intank filter, but white residue

i traded my bf my new filter last week, for his intank, because he has sand, and i didn't want him to spend more money.
so now i have this intank, which i have to run an airstone with, because it doesn't get as much circulation. well, at the mouth of the filter, where it releases the water, there is a patch of white milk-like liquid at the surface. i scoop it every few days, but have no idea what it could be [it hasn't affected my fish so it can't be too bad i hope] Any ideas of what it is?

btw, like i said, an intank wasn't my first choice, i just didn't want him to kill it with the sand