If this is a nightly whiening, then no alarm. Halimeda has footloose chloroplasts (the green stuff) that does a combination of things --they seem to migrate to wherever growth is taking place (to help fuel growth), like the growing tips; and they also tend to retreat from the surface at night, when there's really no point for them to be there (no light, so they can't do the photosynthesis thing) and get eaten by some nocturnal herbivores, like slugs, etc.
With the return of strong light, the chloroplasts come out again and happily soak it all up... turning your Halimeda green again.
This sort of circadian migration of chloroplasts is actually not unusual in green seaweeds ---the difference is that Halimeda has a white calcified armor that asserts its color when the green chloroplasts go into hiding.
Now, if you're talking about a more permanent whitening, and it starts growing this rich, dark green dotlike fuzz in patches on the outside, then you are looking at terminal holocarpic reproduction --sexually or asexually-- and the specimen isn't going to survive the reproductive act of releasing its goods, though its progeny may seed the tank.
If you're talking about a prolonged whitening without the green-fuzz formation, it may be simply dying due to stress. You can try to frag a bit with some green on it and anchor it where it gets good light, but that's a fool's hope.
Next time you buy Halimeda try to obtain the water params at the LFS, and decide if it's too different from your own tank's AND if it's too great a shock to just plunk the specimen into your tank.
I am no expert by any means, but I have had Halimeda in my tank since I started it about a year ago. I notice that mine dies off as it grows like a hedge. The bottom layers turn white and fall off, while the top layers grow tips and expand. So underneith all the good growth on top, the center is all dead = GREAT pod hotel It has done this since day 1 and gorws great!