Ice is not a viable solution. There just isn't enough energy exchange there. Evaporation is the most efficient but if you can't get that working well enough then the chiller is #2.
It's just a concern I have. I might be way off base. OTOH, 1/4 drop could be an overdose. We just don't know if it is extending the life of your fish or severely shortening their lifespan.
I worry about people dosing their fish with vitamin C. I know it is an essential vitamin but there are no published daily recommended amounts for each fish so how do you know the product you use isn't overdosing?
That's just part of the normal lifecycle probably. Most fish can fight it off if they're not stressed but it's rare for a captive fish to not be stressed. Most likely they will disappear for a day or two and then be back five times stronger.
Hermits don't typically hide inside the live rock.
I think it's some type of Mithrax Crab. They are omnivores but have a very strong preference for hair algae. If it can easily catch a fish it will eat it. It would have to be a slow small pretty dumb fish though.
If it were me I would definitely remove it from the display tank. I would be tempted to keep it in a species tank though. When they get a bit larger some of them have some interesting colors.
They hitchhike on rocks in their Hydroid stage. When they get enough food the hydroid will pop off a Jellyfish.
They have a strong sting so can be irritating to fish.
Flower, since you actually require bubbles try saran wrap over the lights. It doesn't cut down on light significantly and once a week you can replace it easily.