They can get some brown splotches or lighter colored splotches when they are sleeping or stressed.
If you're seeing it first thing in the morning it's perfectly normal. If you're seeing it in the afternoon then something is stressing the fish.
When you replace the heater - I have always advocated for multiple smaller heaters vs a single large heater. A broken small heater would have taken a lot longer to get to 100 or probably wouldn't even have the ability to heat your water that high.
Go buy a 30 or 40 food water supply line and then you can tap into where you want. From an RO perspective you're not going to lose much pressure over distance.
Alkalinity look slightly high and Calcium is low. Temp is low for my preference but you would know your animals better than me.
Are you adding something to raise the PH?
Looks like another fish bit him. Or, he was somehow spooked and scraped on rocks.
Any information regarding its environment would help.
That Tang looks extremely stressed.