In the 2nd picture there appears to be two "bunny like" ears on the far side of the slug. Did you notice those when you saw it or is that just part of the background? If they were present, it is most likely a nudibranch. The probelm with IDing those is that there are 1,000's of them.
If you see it again, catch it (wear gloves to be safe), place it in a small container and take some closer pics.