We are going through the exact same thing. I’ve been fighting this crap for at least a month. I even ran my system for a few days with a bleach solution of 2 gallons bleach to 50 gallons water. Bleach does not kill this crap and neither does h2o2.
What I’ve found that seems to be really working right now is a microbial tea.
Take a look at my grow journal, towards the last 25% is really talking about the slime. There are links in there to the original poster of the tea.