these fish are doomed without the help of cyano
is just one visible symptom of overstocking.
that tank is way too small for those tangs. they will eventually get stressed to the point their immune systems will lower and they will get ich or some other illness and then die.
it's definitely cyanobacteria. you don't have to worry about the actual red stuff. you have to worry about why it came.
i would return all but the scopus and the clownfish. and then do weekly 20% water changes vacuuming out what i can with each change.
i would be concerned putting that many tangs in a 120, which is double the size of your tank.
i know that's the last thing you want to hear, but....