I've had this problem in my 30 gallon marine tank for around a month now.it looke like brown diatoms,if you stir the sandbed within 30 minutes it's brown again.water parameters are as follows ph 8.0,phosphates 0,nitrates 5ppm,calcium around 500,magnesium around 1500,dkh 8,I have a turnover of x50.only use ro water which is tested and fine.temp 74f,did have 10 fish now reduced to 6,tunze 9002 skimmer,been running phosban reactor for about 3 weeks with rowaphos,also running filter floss and purigen.at a loss now cannot get rid of this problem only other thing is to do bigger weekly water change.at the moment I change just over 2 gallons every week which I have done for the last 3 months,also have around 7 hermits,3 turbo snails,5 nassarius snails and a couple of small ceriths.just bought a sand sifting gobie.
Not using live rock as I bought dry rock didn't want any nasty critters.also using 30 lb of sugar fine carib sea sand which was washed in ro water thoroughly before use.lights are on timer for only 6 hours a day