Brown Algae!!

Hi guys, today I noticed a brown algae on my live rock
Water is Ro/Di
Salinity 1020
Amonia 0
Nitrate 30
Nitrite 0
Water Temp 27 degrees celcius
Often water changes
No skimmer
2 clowns
4 Cromies

--Lighting 2 LEDs moderate strength 12 hours blue and white followed by 30 mins blue either side then darkness
PH 8

Any cause for this ?
How Do I stop this from happening?
Overfeading?

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