I would say brake slave cylinder, fluid doesn't look dark enough to be axle seal.
Have you any warning lights on the dash, - is the brake pedal spongey, - is the fluid level down in the reservoir ?
I would stop driving it and be checking that out immediately, especially with brakes, that's a fair leak there.
Looking again, it does look greenish on the wheel, maybe the Incredible Hulk pissed up it?
Seriously though mate, it needs immediate investigation, the simple ones as I suggested, or as Shane said, take the wheel off to give an indication of where it's coming from.
When the rear bearings wear, the drive shaft can move in/out and wobble enough to let oil past the seal. Worth changing the seal while you have it apart though. With the wheel off the ground, you'll feel some play if you push/pull the wheel towards the opposite wheel. You can't feel bearing play by wobbling the top and bottom at the rear like at the front because of the rigid drive shaft with its end in the diff. Regrease the bearings as they've had diff oil washing through them.
A greenish liquid huh? When was the last time you topped up your brake fluid? It could be a leaking seal from low pressure levels or brake fluid levels. If it isn't that it would have to be something with dirt and muck in the seal.
Yes by the looks of it, the leaking of the oil/liquid is from the seal and the only idea that I can come up with to undo the screws is to grab your Phillips screwdriver and a hammer and tap the end of the Phillips while it is on the screw. This will send shocks through the screw allowing it to be slightly more easy to undo. I hope that this works. Cheers
Looking at the image, there is leakage, and we have to check well by removing the wheel and see where the leakage is coming from. Last year my car also had the same problem. I went to the Acton service centre there. I told them that since I was facing this problem, they diagnosed the car very well and resolved it. If you want to go, then you can go.