Need help with sand plague

[tprowse0804]

My 75 gallon has been set up for just over a week.

3 days ago, I notied that the sand around the heater was browning and then in the ccorner under may powerhead.
Over the course of the day today, It has taken over a quarter of my tank.
My paramaters are all good (I think).
Nitrate- 20ppm
nitrite- 0-0.5
water hardness-425
alkalinity-300ppm
ph-8.4
What can I do to fix and prevent this?



75g reef
2 clownfish
1 coral beauty
1 orchid dottyback
40# live rock with various algae
trumpet coral
torch coral
And a couple other type of polyp that Im not sure o