i believe the fishes waste had something to do with it.
and i feel it is one way of fish protecting themselves
to have slimy environment in order not to hurt themselves.
what i did is just scrub the glass wall only to make it clear
and let the water current carry out the substance away to the
filter. rock and plants, i just rinse them when ever i feel
like doing it but no specific frequency.
as long as your water is in perfect parameters, you don't have
to worry about it. besides, nobody dare to clean the rocks and
plants in the wild for the fishes...