Just mixing up some right nowI think you pretty much nailed this one.
Shrooms where getting nutrients from food. He stop feeding and they are cupping using energy from light.
I've been flirting with low phosphates in a new tank transfer. Even bottomed out once or twice. Some of my shrooms response where to immediately cup as soon as they smelt chun in the water because they where hungry and ulns wasn't providing enough.
I do think alk is concern. Randy made pretty good point to me that yes my ahroomz aren't using cal and alk as much but all my shelled critters and coraline are. So I quit arguing haha
Regular feeding will most likely fix this for sure.