Black mold on rocks?

[seiz]

Hey all -

Rocks are starting grow some strange black like mold.

Established tank. All tests are good. Phosphates as well.

Also, noticing some algae on sand bed. Maybe cyano?

Any help - thoughts?

Many thanks

 

[melosu58]

I get it every once in a while and I still don't know what it is but when I do PWC`s it easily siphons off the rock.

 

[seiz]

Thanks man.

This stuff is pretty stuck on there. I just tried to scrape a bit off - and not much moved.

Eeek.