What is this thing?

[cillam]

I was just observing the tank as I do and noticed this little critter come out from a hole in the rock.

It looks like some kind of shell-less snail or slug.

Any ideas on what it is and if it's good or bad?

 

[AustinBabler]

I’m about 90% sure that’s a stomatella snail.

 

[Miguel.PT]

Yea it’s a stomatella snail