That guy looks pretty bad, but as long as it's not "melting", who knows... There's a sticky at the top of the forum on the process and dosage.
For me, I use 375mg per 10 gal. I haven't seen anything negative, maybe too much I'm sure. Just what I do. I think the sticky says 250mg per 10 gallons. 125mg per 5 gallons would do the same thing, just foul faster.
Something to keep in mind. 5 gallons of water will foul pretty fast. If I was the one treating it, I would still use 10 gallons of water, and with an anemone in that condition, I would do water changes at 100% every 12 hours, atleast the first day or two. You will know pretty fast if you're wasting your time. And, IMO, high light with a fresh arrival usually will do more damage than good, there's always exceptions, but it's easier to recover from bleaching (lack of light) than rotting (lack of inflating). And, (final one), when my dog is sick, it doesn't like to stick it's head out the window. When it's well, it loves high wind. Something to think about...
Treatment needs to start yesterday with that guy. Best of luck to you.