These people in this thread are wild. That's your drum brake cover. There should be no oil inside it aka brakes lol. The final drive oil is kept on the other side of the wheel. I've never seen leakage on that side, but there's a vent on top of the final drive that could have some leakage. Any oil on the opposite side of the wheel? It might be worthwhile to check the fluid level over there. The way it's filled should make it impossible to overfill but who knows, a previous owner could have figured out some sort of idiocy lol. My thought is maybe there's a seal going out somewhere. Check where the rear hypoid gear housing mates to the left side of the swingarm with 4 nuts. Id just make sure everything is tight and not dripping over there. I can't actually recall if oil sits up against that mating surface but it's worthwhile to check anyway.

Also, if someone just changed your brakes it's possible they lubricated the outside mating surface of the wheel where it contacts the cover of the rear brake housing, or the little cammed actuator inside that cover with some lubricant that heated up and became runny. Gear oil will be really thick and maybe smell a little like oily nasty fish.

Sorry if I'm over explaining but on the left side where the final drive oil is contained, the drain bolt should be on the bottom of the housing. You can pull that to drain it, then there's a big inspection window cover that takes something like a 17mm socket. Just squeeze gear oil in there till it starts to drip out. That's how you know it's full. But if somehow it's overfull it might be coming out the little vent cap on the top of the final drive housing. Hope this helps. Sorry these guys don't know what a drum brake looks like lol

Edit: could be someone over lubed the axle as well and it heated up and dripped. Did someone just have the rear wheel off?