Algae or bacteria on my sand

[didou]

Hi,
I have a 1200 Liters mixed reef tank (with fish), since a few weeks there is algae or bacteria on my sand, it's only on the left part of my tank.

There is a small area where the algae is green, and another area where it's brownish/red.

My parameters are stable from the last 2 months and here are my results from the last ICP test (1 week ago) :

Salinity - 34.2
Kh - 7.6
Phosphate - 0.10 (a bit high, usually around 0.07)
Nitrate - 6.7
Calcium - 419

Can someone help me identify this for me and what can I do to get rid of it ?

Thank you

 

[Gumbies R Us]

How old is the tank? looks like diatoms to me (tank algae that is common with the ugly phase in a tank)

 

[didou]

How old is the tank? looks like diatoms to me (tank algae that is common with the ugly phase in a tank)

My tank is 3 years old

 

[didou]

Up

It's worse now, how do I treat this before it goes out of control?

 

[AlyciaMarie]

Can you post some pictures under white light?