About the lighting. Actinodiscus and other mushrooms are fairly forgiving critters, though they definitely can be over illuminated. Older LED reef fixtures were often missing a good bit of the spectrum that was present in fluorescent or metal halide light sources. It is possible that this is contributing to your issues, but I would not replace anything yet - increasing your nutrient levels is probably a much better bet. Long term, a lot of folks that keep mushrooms and LPS that are highly colored (such as Acans), tend to prefer T5HO for their color rendition and growth promotion characteristics. An alternative is a modern full-spectrum LED fixture such as Radions, AI Hydras, Orpheks, or similar fixtures with a lot of different (controllable) LED colors.
Over the years we get to hear a lot of snake oil about LEDs from people who don't understand lighting science, and we can add this response to the list.
The action spectrum for coral growth is 440-470nm in artificial lighting fixtures (blue). This is true for all types of artificial light regardless of if they are tubes, metal halide or LED.
Because LED makers are adding superfluous colors to try and differentiate their products from basic chinese black bloxes running just cool whites and royals doesn't change biology. The newer fixtures Dkeller is mentioning don't grow corals better than cheaper, older fixtures and this has been established in countless SPS threads on the topic. The added colors along with blue tooth capability and being able to program a sunset in Beirut won't grow corals better, but it helps you sell a $600 light over a chinese black box that costs 1/5 the price but grows coral just as well.
I've yet to see an established SPS tank grow better coral because they have a couple LEDs of mint, 395nm, teal, orange, or any of those other boutique colors invented by marketing depts and not coral growers. There is also no such thing as a 'full spectrum' artificial light source over a reef tank other than maybe plasma sulfur. If you look at the spectrum of a thriving reef tank under an artificial light with my spectrometer it's all blue light with just some smidges of other colors.
Again, what dkeller seems to be saying is LED fixtures with more colors grow coral better, and that has been smashed to death, buried and burned in SPS lighting discussions. Been building reef lights for 10 years and don't appreciate this disinformation. Might as well be making flux capacitor jokes.
If you've ever been snorkling in the ocean shrooms and softies tends to thrive around runoffs and less than pristine areas of the beach and reef. That's because they love less than pristine water. A high light tanks with low nutrients will cause softies to melt in short order, and if you're killing rhodactis it's a really bad tank for softies.
The problem is if you've been running a low nutrient tank for a long time increasing nutrient levels suddenly is almost certainly going to cause other issues like sudden nuisance algae explosions or cyano blooms. It's easy enough to add potassium nitrate (stump remover) or just dose ammonia to beef up nitrate levels to 5-10ppm, but it needs to be done slowly.
Low pH levels, likely induced by low alk levels don't help softie growth much other.