How to get rid of these red stuff?

[Spyd]

You can pick it out by hand as best as you can. Are there any powerheads in your setup? Once you manually remove it, you can move a powerhead closer to the area to increase the flow in that part of the tank.

Worst case scenario is to use Chemiclean but that is only a temporary fix. It will come back if the root of the problem isn't fixed, and when it does, it gets harder and harder to remove.

[Seriak]

And make sure you are using RODI water so that you are not adding any excess nutrients into the tank.