Eggs or sand sticking to algae in a pattern

[Treefer32]

I just noticed today I have sand in strange patterns on just one of my rocks on which one of my hammer colonies is attached. I’ve noticed one of my four clown fish switching from my anemones to my hammers then back for the past week. I thought the behavior weird. But I think this is just sand sticking to the rock in strange designs? Thoughts?

 

[WheatToast]

My Bristleworms create those patterns when they move as their mucus sticks to sand.