Because I have a similar one.

I have several of these guys. 2 are what most people call Serpent Stars (more curl, less prominent whiskers on the legs), and 1 is a Brittle Star (green, fatter legs, more prominent whiskers).

The 1 brittle star has lost 4 of his legs in the last 4-5 weeks. "Normal" molting / regrowth for him is the last 1/4" to 1/2" of each leg. I've had him several years and never seen him lose entire legs.

Recent additions: blue Linckia 3-4 months ago; flame hawk 2-3 months ago. Nothing else different in water chemistry, animals, or anything I can think of.

Any ideas?

Thanks,
Dean