What is this strange pattern on my rocks?

[Saskdevil]

I noticed this strange pattern of stuff on my rocks. I have no idea what it is. I have 2 clowns, an emerald crab, 2 Scarlett hermits, 2 blue dwarf hermits, cerith snails, and a nassarius snail l.

 

[fishski13]

Hey I dont see a pic on here, would be easier to identify.