Most likely just a passerby, taking advantage of some odd niche (found something tasty - from its perspective - on the wall of your bin). Might be a slime mold, not that it makes much difference from a friend-or-foe perspective. You could scrape it into the bin, but just leaving it alone is probably the easiest path forward. Probably will disappear (get eaten by something else) in due course, and reappear some time in the future when conditions right. Probable harmless from perspective of the bin and the worms.