It's totally harmless, just unsightly. You've done your maintainance, tank nice and clean, switch your return pump back on and a sudden flush of flaky gunk spoils all your good work. It clears up pretty quick like you say. I used to experience the same thing and the way i stopped it was simple. Every once in a while, maybe once a month say, take your return line off and clean the inside of it. I thread fishing line attached to a lead fishing weight down the tubing and i have a small rag tied to the other end of the fishing line. A couple of passes with this wet rag and you're good to go.