How often do these get into Fermi Temperate Beach, and for how long do they stay? I would imagine the Dockshrog docks, Mangrovecrystals, and Bonegroves would make it hard to get close enough, given its size.

When it arrives on a beach, it's literally beached by waves and tides. Dockshrogs and mangroves do not occupy the entire stretch of beach and get broken up by flumpus regularly.

What adaptations does it have for withstanding saltwater? Or does it efficiently capture freshwater from the rain? Since it can absorb nutrients from the ocean to fuel its growth, that would suggest it has at least some tolerance to saltwater, so how much tolerance it has, and ideally some vague mention of its mechanism, would be good.
These aren't effectively part-fungus like some crystalflora, so is it really a detritivore any more than Earth plants usually are? Is it like certain kinds of plants that can adapt to the unique substrates of part-decayed compost or rotting but recognizable logs?

I like the detail about the shrog skeletons.

The ancestor lived on beaches and drank saltwater already, so I figure it has the same saltwater adaptations its ancestor did.

Its detritivory is a bit like how saplings might use nurse logs. I'm barely awake so I'll edit clarification later.