I recently had a similair problem. 3 issues here.
1) Theres no such thing as a white sebae nem. It is definately bleached. I had an LFS sell me one recently and even though it lived I learned the hard way.
2) Sebae is a broad term and it could be H Malu, Crispa, or Magnifica. They all require very different things. Good luck teelling while its ill.
3) We need to know what your lights are, what parameters the tank has, ammonia, mg, ca, ph, salinity etc.
How to help; Try to ID the species, its very hard when bleached, nearly impossible but a picture would help. Try placing it at the rock sand border. Depending on the species it will like either rock, sand, or both. Light it slightly more hours than you would your tank normally and feed it every 2 days.
Warning: Some Sebae's have very powerful stings, and will catch and eat most fish. As soon as it was improving mine killed a fish. It wasnt even sticky at that point and still bleached, sting wasn't max strength. USE GLOVES. Neurotoxins hurt!
Your other alternative, and my recommendations if you have anything other than clowns in that tank, is to return it for a full refund and tell the LFS theres no such thing as a white sebae.