The color on your monti cap looks good but that's probably the easiest SPS to keep alive and grow. I'd say the alk spike played a role but the high nitrates aren't great either. You're feeding too much or not changing out filters if they're staying at 30PPM. Is that a 10g tank? If so, two clowns and a cleaner shrimp is somewhat high bioload IMO. I'm not sure what your options are in the rear chambers for filtration, but I'd look into something that will remove nitrates. Small skimmers aren't very effective IMO. Purigen, chemipure, regular water changes and reduced feedings are your best bet. Plus, chemipure elite has GAC which will help absorb any toxins released by your softies that could harm SPS. What kind of lighting are you running? I see some of your zoas seem to be reaching for light so I'm wondering if your light is intense enough.
Regardless, I'd keep an eye on the branching monti and if the tissue loss keeps increasing, you might try to frag it and cut of the dead parts.
