It's just some slime and gunk that worked itself loose from the tubing/canister while it was powered off. No big deal. The air bubbles are just that-- air bubbles that were introduced when the system was powered off. It would be a problem if it constantly burped bubbles, regardless of if you just had it cleaned.
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.
that happens to me too. the last time I used the flexible hose cleaner and cleared out the inside of the tubing going back into the tank before I hooked it back up and there was no white gunk grossing out my tank. I cannot imaging that it is BAD for the tank, but it just seems like gross snotty film floating around. Some of the fish tried to eat it, so i now plan to just put the brine shrimp net over the inflow tube for a little while to catch all that stuff.