I had a similar problem on crypts, my nitrites were way way high so I assumed it was diatoms, once it's advanced I couldn't remove all of it by rubbing, I even used a soft tooth brush on them
It was most likely my tank still establishing itself, my nitrates went back down on their own (I didn't change my pwc schedule), but I had to trim my crypts a little at a time bcuz "infected" leaves were all fuzzy brown and new leaves were coming out clean
Dunno if you have the same thing, BTW I did use some prime to keep my fish safe from the nitrites