The red slime is cyano bacteria, and happens a few months into cycling a tank. Even with perfect parameters, blooms can still happen. Siphoning it out can help, but often it's back in 24 hrs. There are a couple of more drastic steps if you don't want to wait it out.
Turn of lights for 3 days. The cyano is highly photosynthetic, and 3 days of darkness will burn most of it out.
If you are desperate you can use Boyd chemiclean, or red slime remover. Both are very effective, but it will return until the tank is settled in more.
Patience, we have all been through it. It's part of establishing a new tank. Does not mean you did anything wrong.