I don't know if brains spawn, but if they do, then that's one possibility. Do you feed your brain, or have you noticed any fish/invertibrates missing recently that go near the brain? My second guess is that he's essentially "taking a dump." a while after I feed my torch, I see him shrink up and then release huge streams of brown muck that gets blown around a little then dissipates. Gross.
It should be fine, and if it's either, and you've got a good sized skimmer, it'll more than likely be skimmed up right quick.