Hard to tell for sure but anything falling off is not good typically. A few things usually end up down there but flukes typically resemble sesame seeds at the bottom. Flukes are very common in angels and after a few weeks in QT with them unchecked they can do a lot of damage and stress them significantly.
For angels, water quality is a large component for successful feedings. Clams and mussels are great for triggering feeding responses but foul the water VERY quickly. This can be a death sentence, so I use it as a last resort and do a large water change after two days to hopefully nullify it. If it sits in there for a day you can be sure the water is nasty, if it's eaten quickly it will take a few days.
Regarding feeding, if clams or mussels dont work, not much is likely to work.
Formula two flakes seem to work for finicky/stubborn eaters, oddly enough. Live blackworms and for small fish live brine can trigger feeding response due to the movement.
If the animal has flukes, removing those should impact his appetite very positively within 24-48 hours. I recommend a prazi "dip" (follow instructions for a bath on the bottle) and then prazi in the QT 5-7 days apart, you can double the dose for this as you'll want to be aggressive as many of these new flukes are pretty nasty and resilient.