Powder Mildew

That's not pm. there's no cure for it if you ever get it. even commercial grade fungicides you can't buy over the counter are 25-30% effective. Cut it out immediately. Expect an ongoing fight if you ever see it. That does not look like how a typically colony forms. is it fuzzy? it'll grow noticeable in 24h too, so if it stayed the same size, it's not alive. a round fuzzy patch will form. when you can see it, it's already propagating spores.

I don' t see any issue linked to White Powdery Mildew, instead I see problems due to a potassium deficency or maybe an excess of nutes...