Red slime in a corals tank is a tricky situation. Best way to avoid this is to up your flow. FLOW as high as you can with your corals tolerating, and make sure you have a good unidirectional flow going in a circle without any doldrum type pockets.
But once the cat is out of the bag and red slime has set up shop it will spread to areas that have good flow regardless. Now you have to deal with what's currently happening. People use Microbacter7 or Ultralife Red Slime Remover. Either one works, I think Ultralife will knock down the algae in like 48-72 hours. If you use it make sure that you oxygenate well if there are fish and invertebrates in there. Use air stones. My other warning is that corals get freaked out by the stuff so make sure you check which species respond worse than others.