Just thought someone might be able to identify this, I'm thinking maybe calcium? I never saw this before on my hot tub filters but I also never used a stain and scale preventer until recently. My hot tub is on a back deck, gets light use, I only use liquid chlorine and sometimes a bit of muriatic acid to correct the pH from time to time. I drain it once a month, it's only a small 200 gallon inflatable. Photo attached.