Strange markings on urchin shell

[JayM]

I’ve had a couple of urchin shells in my tank since the beginning (3 months or so). My Tiger Pistol Shrimp drug one into his lair about a month ago and haven’t seen it since, and today I happened to notice some strange “rings” on the one that’s regularly inspected by my hermit crabs. Are the “rings” normal discoloration? Or is some strange critter growing on the shell?

 

[Rtaylor]

Probably coralline algae or just calcium deposits

 

[WheatToast]

Probably coralline algae or just calcium deposits

+1. "Green coralline" tends to establish itself before pink and purple coralline.