Have you noticed anything different in your stony corals? Is your leather a sarcophyton? If yes to both, you could be witnessing chemical warfare. That's just a guess, there are all sorts of possibilities. Could be adjusting to the light.

Your algae is almost certainly related to the light increase. I'd cut back on the photoperiod and get the siphon out to remove what you can. Probably the hermits won't do much except get whatever they can from your sandbed:> Try a diadema urchin to graze on algae, although that really only works for some types of algae.

HTH
Matt