Diatoms on SPS tips?

[dcw1sfu]

Have two frags in my tank that look like they have a small dusting of diatoms on the tips. It can not be blown off with a turkey baster and not sure if it's harming the corals. Showed up approx 2 days after a 20 % water change. I have no other signs of algae or diatoms in my tank and in fact I only have to clean the glass every week at this point.

Paramayers as of this morning during lights out:

Nitrate between 5-10 ppm
Phosphate .01 Hanna ULR
Calcium 420
Alk 7.3
Mag 1300


Here's a pic if you can make it out.