it's a bacteria as far as i remember, it grows wherever it can whenever there are excess nutrients. You will always have it to some extent, that cloudy crud you scrape from the glass or the slight film on all of your ornaments. If it gets a food source it will bloom. Clean it off and do a water change, about 50%, clean down your media. It's not a huge problem but it doesn't look wonderful.
Source: oh boy i've had this plenty of times
I did remember doing an experiment once where i put an algae wafer in and lent it up against a small colony of this stuff. Lo and behold, the next day it was enveloped in a blanket of this translucent white organism.
EDIT: Oh, if it gets into your filter foams, good luck getting it out. P.s, it's a nightmare.