Not condoning anything here, but I have thrown nems in newish tanks years ago and they were fine. Bash away if you like, but I have over 30 nems now and all are doing well, granted they are not thrown in tanks on whim anymore lol, but currently have 2 in cipro as they got delayed a day in shipping and looked bad. Note: I do not treat all nems, I only treat if needed.
So with that said, here is what I have seen with haddonis. I have rescued quite a few from fellow reefers who did bad things and about killed them.
They are tough and if not sick can handle this. Again, not sick, but need some questions answered to determine if sick.
So here we go with the observations and questions, as what is done is done. No going back, just forward and towards a fix.
Observation one: nem was bleached in August.
Observation two: nem does not look bleached now. That is a good thing.
Question 1: what does the mouth look like? Closed? Not gaping? No “guts” sticking out?
Question 2: all of the rock and such came from the old tank right?
Question 3: Is the nem inflating and deflating or just staying small?
Question 4: how much tap water did you use? Nems hate chlorine and chloramines. It seriously ticks them off.
Question 5: do you have a tank or container to put the crab in? Ticked off nems do not need any added stress. The crab may irritate things more.