Cone shaped shell

[PurplePolyp]

This cone shaped shell appeared on my live rock about 2 months ago. He's got red and green algae on him but so does this rock. For a long time I thought it was part of the rock. Then one day it disappeared. Now it's back on a different part of the rock. What is it or inside it?

 

[Convict2161]

This cone shaped shell appeared on my live rock about 2 months ago. He's got red and green algae on him but so does this rock. For a long time I thought it was part of the rock. Then one day it disappeared. Now it's back on a different part of the rock. What is it or inside it?

Snail maybe or hermit crab?

 

[mr_X]

It's a limpet. Harmless and beneficial.