Moved to “sick fish or coral” forum:
What are your current water parameters ie: temp, sg, ph, nh3, no2, no3, alk, ca? What method and how long did you acclimate? Did you expose it to air?
I suspect your brittle star was/is infected and its health impaired from collection, shipping and handling... It will either self-heal or perish... if the latter, remove the remains from your system.