theres been a thread for this already somewhere. dont worry about harming the fish with such residue. one betta even ate the stuff and had no ill effects. if i recall its some harmless bacterial or protein residue, gotta find the thread for it. just wipe it off when you do a wc and repeat it when it comes back.
though i do find some other issue with the 25w elite mini that i need to email hagen for. seem slike the teflon seal for the head slips out from the control head. the thing still works well though its just weird.