Help with Id'ing white spikey thing

[mitchrapp]

Does anyone know what this white spikey thing on my rock is?

 

[Sk8r]

Hermit crab in a shell?

 

[mitchrapp]

Definitely not a hermit crab. It doesn't move. Maybe a snail, but I've never seen it before and suddenly appeared.

 

[Sk8r]

It's uncommonly organized for a sponge, but might be.