Agree its diatoms. Most common in new systems with a fresh supply of free silica. Fortunately for us most silica is NOT consumable by diatoms.
As a comparison, we are a carbon based life form and all good food is as well. We consume carbons and we grow and make little ones. Diamonds are principally carbon. We cannot consume the carbon in diamonds no matter what we put on them. We can eat them and poop them but we will get nothing out of it except a doctors bill.
Most silicates we see are very tightly atomically bound structures. Glass, Quartz and silicon carbide are very very common, but, like diamonds, completely useless to the ugly brown diatoms.
Free silica that the diatoms consume are much more rare, except, sadly, in coral sand. Coral sand is a part of the silica circle that includes diatoms and corals. So new systems with a coral based media will almost always go thru a diatom bloom. Oh, still the sand and BOOM, new silica is made available for another bloom.