mslfl wrote:

1) Does anyone know what this substance is?
2) I have not added fish to my tank yet (still fishless cycling) but
is this substance harmful to fish?
3) Is there anyway to stop this substance from coming back each time I
clean the diffuser/counter?

This stuff occurs regardless of the diffuser system, from what I've
read. It has been known to clog an airstone, causing the CO2 bottle to
over-pressure and explode.

It's a fungus or a yeast, according to posts I've seen. My fish have
been known to actually nibble on it, without any ill effect.

As for getting rid of it, I doubt that you can. Best thing to do is
put a plant in front of the diffuser so you can't see it

--
www.ericschreiber.com