Strange snail-like creature


Reefing newb
Does anyone know what this guy is. It is amazing the things Im finding crawling around in my tank now that all the predators are gone.

It moves just like a snail but has no shell
I think thats it. The one I saw was more of a dark charcol grey color but the description sounds right. Cant believe I've never seen any before. There not exactly small. He was just crawling around in broad daylight, but so are all the other creatures that normally hide. I hope they have sense enough to go back to their normal behaveior when I put the fish back in. This is there chance to be friutful and multiply so maybe some will servive.