Is this what I think it is

Looks like mineral drips dried on the leaves. Do you spray anything on the plants what so ever? Or so the leaves transpire a lot and develop water drops on their tips that fall off?. If it rubs off/or doesn't rub off AND is grainy like salt/sand it is not WPM. If it rubs off and its soft and powdery like chalk. Its is highly likely it could be WPM. WPM thrives when the due point is hit. Try and avoid it at all costs.