white slime/fluff outbreak?

Hi all

I have recently set up a new 20 gal tank in my lounge,
and have been doing a fishless cycle for a 3 weeks or so (adding a little flaked food every day)
plus added a few litres of old tank water from my other tank, to get the bacteria kick started

I managed to get the amomia down to 0, nitrites down to 0 and nitates to around 4-5 ppm (orange-red on chart)

at this stange i introduced some brand new (and washed) gravel,
a week later, all doing good and parameters stable (no water changes though as no fish in there)

then a week ago i added some java moss, in the last few days i seem to have an epic outbreak of
some kind of white slime , it looks semi white/transparent and has covered all sides of the glass, and breaks up floats around when I tried to scape/scoop with a net. I have noticed prior to disturbing it on the glass, that it was also growing up in patches from the gravel, again like a fluffy/cotton/slime growing in clumps.

Would anyone know what this is and how to remove/ avoid?

Im guessing its a bacteria, and possibly due to me feeding the empty tank (in an attempt to cycle) and lack of waterchanges, but it looks terrible and a nightmare to clean (the old tank water i used might play a part, however the tank from which it came is fine)

Ive actioned a 50% water change, and tried to “hoover” the patches from the substrate with using the “disturb with finger and suck with sython” technique with varying amounts of success and tried to scape the glass and net of up the slime, but still a lot of floating slime which almosts clouds the water.

im glad i dont have any fish in there yet to worry about, its just frustrating at the moment as it looks terrible, especially when the slime hangs and mats into the newly installed java moss.

any help/ advise would be massively welcomed

(quick after thought, i did initally add some treta easybalance, and maybe a slug of tetra safe start)
maybe a bacteria overload ?