That is a red euglena bloom in the pic. It occurs in our pond when we have a heavy bloom of plankton. Can't say what happened to your pond because of all the factors you mentioned plus there are a number of platonic algae that can be red.

If you had a strong plankton bloom and added dye and copper you may have come close to an O2 depletion when you reduced the O2 producing plankton.

Why did you add the dye and CU ?