I had an outbreak of this last month that I just couldn't get on top of. I was syphoning it out daily, doing increased WC's, increasing flow and decreasing tank feedings. The red stuff would not go away.
A member came by my house to pick up something, saw my tank and said... "don't tell the purists I told you, but try Chem-Clean"
I'm not recommending Chem-Clean as a regular preventative of red slime (it treats the symptom, not the cause), but it's a miracle for zapping the junk from your tank when its gotten completely out of hand so you can get a handle on things.
I followed the instructions on the container (treated ... waited 48 hours ... repeated treatment), and the stuff disappeared.
I then adopted more disciplined tank maintenance, and I've not had the problem since.