I got the same thing on a Java Fern in my Betta tank after using a medication that contained Melaleuca. The relationship could be coincidental, though. It is the only plant in the 1/2 gal
Betta tank. The Betta is the lone finned wonder in said tank.
I took the Fern out and rubbed all the leaves with my fingers under running tap water. The chalk came right off, so I replanted the thing. Almost two weeks later, now, and it has not returned.
Water parameters have not changed. Only difference I can come up with is that medication.
Two weeks after the scrubbing: