I laugh at Hole In the Head.
Join Date : Feb 2009
Location : Sugar Land, TX
Posts : 1,686
Thanks : 174
Thanked 145 Times in 120 Posts
Whenever I've cleaned a filter, only to have it spit out white looking crud when I started it out, what I've seen is the filter pushing out the crud that had lined the outflow hose.
Cleaning the filter allows it to run at a higher flow rate (it's not as clogged as before) which applies more pressure to the outflow of water, and that higher pressure pushes out crud that had accumulated over time. It has never harmed anything (my fish even eat some of it), and the filtration just cleans it out (but I have very high flow).
I can't say you're getting the same white crud, but it's possible.