Could be the mucus lining of the intestines that it pooped out. Stress causes hex to flare up. Try soaking pellets in Epsom salt solution 1tbsp per liter water. You can add a piece of smelt or shrimp to the solution for scent to entice it to eat.
I've used pipettes and syringes to inject down throat into stomach before on fish that won't eat with bad hex. Epsom is incredibly safe, I like it more than Metro.
A user named RD has a great sticky on this in the disease forum. You can't find it easily on mobile app without using the search function. On computer it will be right at the top of the screen in that forum.