Denanbob, it does look too white to be dirt - probably is DE.
I think it is (was!). We vacuumed it up and then it was back the next day but not as heavy. Waited a couple days and vacuumed again last night. This morning there is not a speck to be found. I imagine it was just excess DE that the filter didn't need. I was worried my filter was malfunctioning, but now I think probably not.