IF you got him bleached like that I would try to bring him back or send him back if possible honestly.
Since this is your first nem really not fun to try to get a sick one and start there.
BTA's can be very hardy, What else do you have in that tank?
The reason nems are hard to keep in young tanks is just STABILITY. young tanks parameters tend to swing and flucuate much greater than an established tank.
If you can however work and keep your water params stable you will be fine. But this is a really hard task as well for a budding tank and a new reefer.
Water quality and stability is super super super key for nems, they inflate their entire bodies with the surrounding water, so if you have high NO2, NO3, or NO4 you're going to have problems with the Nem since they're toxic to him in high numbers. (there's also many, many other things in your water that could be wrong, but this is a common example)
After feeding and other things in young tanks, those parameters tend to spike and flucuate often. All that flucuation will cause a lot of extra stress on the nem thus leading to it's death. More so than with fish or other animals since it will be constantly trying to adjust the water in it's body to match the surrounding water, however if that water is also toxic it's just like triple stress on the nem.
I wouldn't feed him until he's healthy, it will add excess nutrients as well to a tank like this that are unnecessary right now. Nems don't need to be constantly fed.
If you insist on a nem right now I totally understand but at least try to get a healthy one.
Go to google and type in Healthy Green Bubble Tip anemone, you will see how he should look.
This isn't a complete understanding of how nems work, but just a laymans explanation which will give you enough insight as to what to do to help keep a nem.
