Hi AZ Kid and welcome to The Reef Tank!
How long have you had the star?
Brittle stars are pretty hard, much more so than the linkias, and more than likely it will make it through this infection. It may lose the limb but it will grow back. If it looks fuzzy or is starting to spread though, you could dip a Q-tip in some iodine, pull the star out of the water and dab it with the Q-tip. Doing this twice a day for a few days may cut back on the infection.
Target feed him to make sure he gets plenty to eat, that will help his recovery.
Alice
-oops, that should read "Brittle stars are pretty hardy!-