If its a blue & yellow damsel fish, they will turn a darker blue & hide in a corner alone. If you have any other fish they might start bullying the poorly fish & pushing him/her away from food because the poorly fish is weak. To prevent the damsel from starving, put a bit of food next to them where the others cant steal it.
This is happening to my damsel fish & the clear part on his tail is either being bitten off from the other fish, or its shredding. I dont think there is any way of stopping the bullying, but you might be able to get some medicine for the illness. Just keep an eye on the damsel & if anything unusual or strange happens go & ask about it at the shop where he/she was from.