Anyone knows what this is? ID?

[hkreefjunkie]

Hi all!
I recently found one of these sort of snail or something in my tank. Dunno what it is! Anyone know? It mostly comes out at night. I saw a big one and then 1 week later, I had like 40-50 of these babies crawling around my tank! I guess it spawned or something without me knowing! Would most appreciate anyone letting me know if it's good or bad for a reef tank! It looks harmless cuz I see them grazing on the glass and live rocks for algae.