sand bed is brow :(

[Palluk33]

for the last two moths, my sand bed has something on it I try to remove when doing water changes but is like a thick carpet any help what is this stuff?
nitrate 3ppm
pho 0.1ppm
amm 0.0
nitrite 0.0
sal 1.026
akl 9.5
cal 430
mag 1480
a reef tank is 5 months old, thx for any help

 

[Hitman]

Part of the ugly stages of your tank maturing. Is that a typo on your Phosphate? .1 is high in a young tank and can cause your algae issue as well. Most of us prefer to keep it closer to 0.02

 

[Kid_Vegas]

Do weekly water changes by suctioning your sand bed.

 

[EW_Fish]

Increase the flow to help move your sand a little bit. Do 15% water changes

 

[vetteguy53081]

In addition to above recommendations, reduce white light a little, add 1ml of hydrogen peroxide per 10 gallons daily and reduce amount of food fed.
I assume you’re using RO water?