OUCH, looks like Cyano or Spirulina to me.
Both are ugly. @twilliard can tell you more about this, how to know which is what and a treatment game plan.
Yes I agree cyano, and based on the other algae growth you might have an excessive nutrient situation going on. Test for nitrates and phosphates and increase flow to those areas most problematic.
That is what I was afraid of. I watched the video @twillard has on how to treat and I have a few questions. Do we need to remove the corals and fish before starting the peroxide treatment?
What side effect should we watch for if they stay in? Will the peroxide kill off any of the beneficial algae?
Based on how @twillard describes how to identify the type it is classic cyano unfortunately.