The brown cyano may be dinoflagellates if the snails are dying.
Do you have a calibrated digital pH meter to test pH? Test kits are useless for pH.
The mini crash suggests you're dealing with nutrients from die-off. Temporarily (or permanently) adding extra water flow to stir things up better would help a lot. Especially blowing behind rocks and down by sandbed. Maybe look into vodka/vinegar dosing or biopellets until you're under control again. Some Lawnmower Blennies will eat prepared foods (most won't), you could look for one that will. Or a smaller species Foxface Rabbitfish if you have room in the tank. Be diligent with weekly 10% waterchanges taking the time to pull out the algae (12-18" tweezers are handy for this purpose).
What is Mg at? Do you dose Mg? If so, which product?
In the end, elbow grease and time.