PLEASE PLEASE Help Me Identify this thing

[180dude]

I was looking at my tank with the lights off and found this creature. Do you know what it is? I'm thinking sea cumcumber, but I need help. It blends into the rock real well and is appx an inch long. It has a hard shell and moves slow. I would like to know what it is and if it's bad.
Thanks

 

[aw2]

Looks like a Limpet to me.

 

[180dude]

Are they bad? Do you have any info on them?

 

[viper_930]

It's a chiton, a harmless algea grazer.

 

[180dude]

Your a genius! Thats exactly what it is. Thanks for the help.