OMG, I feel so bad for you. Everything you are doing and everything that is happening is exactly what I have been dealing with for over a year and a half! I have spent more time and money on salt, equipment, new lights, pumps, ro/di crap, lights out,etc, etc, etc. to no end. Levels are all "good" , ro/di .01 tds or less. My fish have done awesome through all of this, until now. About 3 weeks ago I thought my rocks were finally looking better and gha is mainly growing only on the bare bottom, but now my fish look terrible and are fighting the start of HLLE and fin rot, and who knows what else. When you figure this out Please let me know what worked for you!
NOPOX created that nasty slime crap for me too. But I do believe it is what is finally helping me kick it. What I did was cut back the dosage to about 20% of what I was using , and dose it Off and on. I will do it for a week or two at a time then stop for a week or two. The heavy green is almost gone (1 or 2 rocks yet) and the slimy stuff has not ever come back with the use of NOPOX at this lower dose.
Because I have not been using NOPOX for a bit I decided to start with carbon again, (havent used carbon or gfo for a yr and half, because NOPOX says not to) and within a week of that use my fish all have developed issues. Coincidence? I don't know , they looked beautiful until that usage. ugh! Good luck to you my friend!