Opaque growth on rocks

[r1c4y]

Hey guys, there's some sort off growth in my aquarium would be great if anyone could identify what it might be.

 

[Timfish]

Sponge, beneficial. You likely have more back behind your rocks. Cryptic sponges which prefer low to no light are essential recyclers in reef systems. Many feed of the labile DOC produced in systems and can promote pathogenic shifts in coral microbiomes and convert it to DIC which corals use.