Sand sifter stars rarely take to spot feeding, and most will starve to death in our tanks in 12-18 months unless it has a large sand bed.
It consumes the beneficial creatures in a sand bed and then slowly starves without any apparent signs of trouble. The arms then begin to disintegrate, and most people think they have been killed by crabs or something.
These are sand bed animals and IMO, being on the glass is not particularly normal for them. Success is measured in years, I'm afraid. they can survive in large tanks (again with open sand beds) but in most tanks under 100 g they do not do well long term (over 2 years).
I never, ever, recommend keeping them.