Edit: Nevermind what I wrote below, I'll leave it but based on more info, this reply seems non-applicable.
What are you referring to by "AC"? Typically that means air conditioning. [/edit].
There is a chance it is hydraulic fluid from the shock absorber (strut in your case since it has independent rear suspension) but as likely the brake caliper is leaking. Cost depends on which and whether you DIY repair or have someone else do it as the labor will cost more than the part.
Jack it up, pull off the wheel and see where the leak is coming from. If there's a lot of fluid sprayed around then you may need to clean it off and wait for it to start to reappear to find the source, though if it is the brake caliper then the brake reservoir under the hood is probably getting low, do not wait to check that and top it off if low. Do that before any further driving if at all possible.
Green is not a typical color for brake fluid, but there is at least one manufacturer that makes multiple colors and some theorize that some copper/brass in the brake system can corrode to color it green. "IF" the fluid in the reservoir is the same (depth of) green color then I would assume it started out green.
If it is closer to clear in the reservoir but leaking green out of the caliper then I would consider flushing all your brake lines with new fluid after doing the repair, continuing to refill the brake fluid reservoir before it empties and repeat until it comes out clear at each wheel caliper.
If it's a strut, I'd consider replacing both rears at the same time to maintain better balance of handling, plus the other would tend to have the same amount of wear and might be due to be changed anyway.
Cost depends on where you take it if you're not DIY. Figure they'd charge an hour of labor (though some places will give you free labor if you have 4 struts put on at once, a sale/promotional kind of thing) but then you may need to pay for an alignment, especially for front strut replacement.
Anyway an hour of labor may be $60 to $100, then around $100 for a pair of struts, a little less if bare strut or closer to $150+ if they are complete with new springs and top. A brake caliper for that is around $50 to $70 plus a refundable core charge deposit, plus you might need new brake pads ~ $30 if much fluid got on them.
If it's not the caliper but instead the brake line itself is leaking from damage or rust (rarer to have this amount of rust with it only being 5 years old but I suppose it's possible if used off-road or in winter salted road conditions) then if it's the flex hose at the caliper it could be cheap and easier to repair, maybe $30 or less plus labor. If it's the steel line before that, it depends on who does it and how much they want to replace, could be $100 or several hundred dollars but usually a leak from the line before it would not leak behind the wheel.
Start by pulling the wheel to see what's what. All prices above assume shopping around for the best values. If you just take it to a dealership they'll probably charge you over double those prices.
With all due respect, I think you are wrong. I looked up the lines, they are rear air conditioning lines for the back AC . The green fluid is the dye the dealer put it to test the AC system because it wasn't working before I bought it. It explains why the AC wasn't working. All the freon gas is leaking out.
I suppose it's possible but you made no mention of anyone putting green dye in to test it previously, nor that there was a problem. Wouldn't it be good to give us ALL the info before wasting that huge wall of text I typed?
If the dealer put the dye in to test it, how could they not have found the leak and given an estimate? The prior owner made no mention of this?
I can't give an estimate for rear A/C lines, never had a vehicle that had them fail, didn't even realize there were any that went back there on an Explorer. You might post a picture of where it's leaking in case someone else knows the parts and labor breakdown, or find out who put the dye in and call them to see if they recall what the repair cost would be. It is curious to me that someone went to the trouble to have that checked but not repaired as non-functional A/C on a vehicle that young would tend to offset the resale value a lot.
Sorry about that. It's back at the used car dealer now. I know they knew about it. I don't trust them now. They said they give a 30 day or 1000 mile warranty on all their vehicles. I trusted them when they said it's in need of freon. I figured they would test the system. They do have a good reputation, and they haven't given any flack about bringing it in. I'm waiting for a call and I'm hoping it's fixed. You never want issues when you buy a vehicle. Thank you for your response.
If they won't fix it cheap or free, I'd shop around as you're going to pay the highest price possible at a Ford Dealer. For example the rear evaporator is $70 at Rock Auto but probably well over $100 if not $200 at a stealership. It could be something as simple as needing a replacement o-ring and most of the cost is the refrigerant charge and labor that any place which has the A/C machine can do.
On the other hand if a custom shaped line is damaged and nobody wants to touch that job because that part isn't available on the open market ready-made, then yeah a dealer may be the only one that can get the part... sometimes a shop "can" do a repair but doesn't want to bother if they already have enough other business, especially if it's a lot of real labor to custom make lines.