I have had mine for a few decades. It does have times of good and times of bad. Some years it is fully expanded and huge. Other years it would slowly shrink in size to half. But I have never had it wax up and i always get it back up in size.
It does prefer living in the substrate and not on the rocks. It likes a thick amount where organics build up. As one of it primary feeding modes is uptake of dissolved organics. They typically come from lagoon or sea grass areas so tend to like supplemental feeding of small crustaceans. Just based on mine and looking at the areas it naturally comes from I bet a portion of its diet also includes phytoplankton and to some extent zooplankton. I rarely feed mine directly. The only time I do is if I have continued months where the tissue is shrinking then I will feed it larger things like cut up shrimp soaked in Brightwell AminOmega orAmerican Marine Selcon for a few months till the shrinking stops. Mine like flow but not across the base where it lifts it up. I create a situation where say the flow is coming from the left side of the tank towards the right. I place a rock on the left forcing the water to go up and across the tentacles and not lift the bottom up. This also creates a area of low flow so organics and other waste like poop and uneated food builds up. I do use a turkey baster on this spot once a month to clean it out.