that is usually where a rear main leak will show up but check above it for signs of a leak from somewhere else too. oil leaks can trickle down from other places & collect there too.
if it is indeed a bad rear main seal but the leak is not bad you can try a high mileage oil like valvoline maxlife or other brand, they will usually help a small leak or seepage. & they do not swell the seals or cause them to harden etc as some may think. there are also some good additives you can try that help with minor leaks but i would try a HM oil first. also check the PCV valve isnt clogged creating excess pressure in the crank case.