Could be a byproduct from diatoms, yes. Could also be a little cyano, or could be some other gross algae.
New tanks are gonna go through this. I went 8 months without seeing anything really ugly in my tank - a diatom bloom on the sand that lasted maybe a week - then I started dosing various things, including phosphate and nitrates, along with adding more fish and feeding heavily.
Mine is finally going through some uglies, but I think it’s hit a plateau after I’ve been experimenting with a few options.
Listen to everyone when they say “be patient” (which also ignored in my early 20s) or you shall suffer the same fate and look upon what thou hath wrought upon... thyself.