ID Please

[michaelchen]

I have two of these, both appear about three weeks ago, at very small size, and now it just keep growing fast and moving faster at night. I can see that there is some sort of shell on top of it. Can someone help me ID it? Is it good or bad?

 

[Deanos]

Stomatella varia sp.

 

[michaelchen]

Thanks!
I think those are good keepers, are they?

 

[daaznmofo]

they're good