Then Church nailed it, it's just biofilm that got loosened up.
And purging bubbles for a little while after startup is normal since there's usually a bit of air that gets trapped in the filter/filter media and has to work its way out. If it keeps bubbling past a few hours, you may need to check all the seals to be sure air isn't leaking in somewhere, though. My XP4 usually bubbles all night but the air has all worked its way out by the following morning. That's just from air getting into all the sponge media when I squeeze it out to clean it.