Identify the brown algae on sand

[oualid]

Hi

I am wondering if its Diatoms or Cyano on the sand.
Cyano can also have brown color I suppose.
The aquarium is mature, about 4 years old, it shouldn't be Diatoms.
Otherwise the aquarium is healthy.

Any idea?

 

[CC13]

Following along.

 

[ajmidget]

Looks like there are some stringy bubbles coming up from your substrate, perhaps it's Dinos?