Help ID this Glass Sucking Creature

[RussWalters]

I have recently dropped live rock from TBS in my new tank and I have noticed two of these guys sliding up the glass on my tank. Any help on ID'ing them and letting me know if they are a nuisance or not. It seems like they have a bit of a hard outer shell, but I can't tell for sure.... I knocked one off the glass and it just headed right back there and re-attached itself.

Thanks in advance!
http://uploads.tapatalk-cdn.com/20160716/7b7114f0d5bf4518c2c0d9187564e322.jpghttp://uploads.tapatalk-cdn.com/20160716/450deb23697bf3d1e6bc1a31fc7a24d0.jpg

[jraker]

Looks like some kind of limpet.

[Ron Reefman]

Looks like some kind of limpet.

It's hard to say based on the photos, but I'd say you are right.

[Dogshowgrl]

Limpet. Good guy.

[SeaCucumberFan]

I'd say limpet

[RussWalters]

Thank you all. I think we can say case closed on this one. Limpet it is.