Ya I believe it. I thought the crab had something to do with it but after heavily googling the only thing I could find was "porcelain crabs will not harm the anemone" I just dismissed that as the cause.
I don't see how even a large rbta would put up with something stealing its lunch right out of its guts, the relationship is clearly not symbiotic.

I have noticed that even clownfish can be really hard on their host anemones. Mine will steal food out of the anemone everytime I feed.

It's definitely not symbiosis in my opinion. It's clear-cut parasitism that is usually mostly benign but not always.

Maybe rehome the porcelain for now and see if you can get the anemone to start feeding again? Maybe start off with smaller mysis until you see it getting bigger?

My RBTA is host to both a porcelain crab and anemone crab (the clowns got evicted with the fallow period for crypto and couldn't get their spot back), it can extend to about 8-9" across when fully inflated. That seems to be a happy size for it, but prior to that when smaller and before the clowns became hosted, the RBTA and porcelain crab were not a good mix.

That should have been anemone shrimp, but I can't edit the post.

slowly opening again
 

Well it is very slowly opening up again. I think it will take a few weeks, but it looks a lot better already. The anemone crab in the sump is just hiding and not actively filter feeding or anything. I may have to get one of those pink silicone anemones from wally mart for him.......
https://i.imgur.com/nsPonzzl.jpg

The porcelains don't need an anemone to be hosted in, they'll perch on pretty much anything as long as there's a current to catch food in.

Stop feeding so often and quit adding so much stuff into your system.

Jardini

Hey great advice..... you must be from Edmonton....

my RBTA is exact same size as that showing in picture , had the same problem after a while she moved away from light
now the color is coming back and looks healthy

pix
 

I'm pretty sure the problem is the porcelain crab. The anemone is opening more and more every day. I found a few pix from 7 months ago that show what it did look like.

Was about 4-5" across when I got it
https://i.imgur.com/MFM6xP4l.jpg

Pic of it spitting out a half digested yellow watchman goby
https://i.imgur.com/VvE69mQl.jpg

The day I got the porcelain crab
https://i.imgur.com/bPWlnyCl.jpg

What it looked like a week or two after it split. The small one on top is the one I still have. Bigger bottom one was tossed out in fear it may die while I was away.
https://i.imgur.com/MiWSVjJl.jpg