weird looking snail

i don't have photo of it because i don't know how to post it. but i have some type of snail without a shell, it has brown lether looking hump where the shell must be. he is all brown. it's a second time i saw it and it moves prety fast for a snail. he also have antennas on its had. i was just wondering if it can be harmfull to my soft corals or fish.

:thm: you got it!!! thank you for such fast respond. it is a slug. when i saw it for the first time i got scared, it looks freaky. from the info you sent me i can see that he is a good guy, great algae eater, which is good. i'll keep him, but hope he won't have babies, because he is too scary looking. i thought that he is some kind of leech at first but antennas made me to think that it's something else, but never thought of a slug.