Cycling issue

[Hairytank]

So after four years away from reefing I am getting back in... my wife actually was the one who suggested it...but I am have a weird thing happen while cycling my old rock.

Background: Bleached and acid washed my old dry rock and have been cycling it in a Rubber maid container for just over three weeks. There are several pumps maintaining a high flow rate. Using Coralife Marine salt mix to cycle the rock. The temp is 79, specific gravity 1.025, Ammonia 0, Nitrite 0, and Nitrate 5 ppm.

Issue: Three days after weekly water changes a significant number of small white particles are floating on the surface (not alive/swimming) and there is a mildew/musty odor.. I had a marine tank for over 10 years prior to shutting it down and never had anything like this..

Any thoughts?