Yeah I've kept Anthias before and have seen stringy poop before too, I was just checking here cuz there are so many wise reefers here to discuss things with. I believe myself its constipation. It has passed the long white feces out and now has a miniature grey one hangin out for all to see lol.
Lol. Quite the rainbow of feces ya got goin on there!!
Stringy poop is usually a sign of internal parasites (worms) and is quite common in anthias. Prazipro will cure it.
Aquarium Fish Medications: Aquarium Solutions PraziPro