star fish dieing?

[mws]

+1 on bacterial infection, don't think you can do anything.
But I wondered if cutting the leg up to a healthy body would stop the infection without killing the starfish. It should be able to grow the leg back.
I think at this point it worth trying anything, as it is a goner.

[naesco]

No it is dying as most starfish do. They simply deteriorate as shown in your pic,
The exception is a blue linckia starfish.
If it is slowly acclimated, is healthy when bought and placed in a large, mature tank with lots of live rock it has a good chance.