Help with algae identification

[MrGoodbytes]

Looks like you have dinos (dinoflagellates - the red strings). I had a battle with these earlier this year. There are a few methods that people suggest to rid aquariums of these but I don't think they ever fully leave. Make sure you have zero phosphates for one. I doubt you have anything in there that eats them. A GFO reactor would probably help. I didn't have one so I ended up tearing the reef apart, baster blasting every rock in tank water, and reassembling. A 2 day blackout may work also. Some suggest increasing the alk to higher levels like 10 or above. In the end, after about 6 months of fighting, they diminished when I got a decent skimmer but have never 100% vanished).

You mention things came from another system but you didn't mention if the rock did, which is what's most important (cycled vs uncycled).