Yeah - agreed. This is likely mostly bacterial slime. I recenlty cleaned out a Velocity T4 at a friends installation - the impeller was covered with this stuff. We used a tooth brush and scrubed the whole inside of the impeller housing and impeller. When we started the system up - voila - very little 'white stuff.'
So you may want to consider cleaning out your pump - the white discharge may be an indication that it's time.
Amazing how tough those bacterias are considering they stick to that impeller and the thing is turing like 1750 RPM - Yikes - how do they do that.
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein (1879-1955)