To elaborate on Myka's post ...
I find I get your kind of dust algae when my NO3 and PO4 are out of balance.
A recommended 'ideal' balance is around 1ppm NO3 and 0.03ppm PO4.
I never used to worry about these #'s, but found I was having issues with my PO4 being high and my NO3 always being 0.
After reading a post from ryanerickson about boosting Nitrates, I looked into it further. Then I proceeded to damage my corals by adding Nitrogen at a level too high and too fast, which caused my PO4 to crash.
I'd recommend you try adding a 'very small' amount of an NO3 source, such as Seachem's Nitrogen, and see what happens, per se.
Start with about 20% of their bottle dosage every other day, and test/monitor things.
Test daily to track what levels of Nitrogen addition affect your PO4 without completely reducing it.
DO NOT add too much Nitrogen quickly if you have corals. This will guarantee a crash in your PO4 level and result in some/all coral deaths.
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