Been cyling my new 125l tank for a week now with some hardy fish and the last few days this kind of white stuff has started appering. It looks like the white stuff on dandilions that you can blow off.
Is this on the fish or in the tank? Giving us a lot more info on your situation like the type and number of fish you have, recent ammonia, nitrite and nitrate readings, temp of tank, any chemicals used etc will help a lot .