Damselfish lack the digestive enzymes to process many kinds of algae, and members of the genus Polysiphonia are some of the few they can eat. The fish also act the gardener (Mr. McGregor, perhaps) in chasing away invaders like the sea urchin, the Peter Rabbit of the damselfish garden.

What does a damselfish eat?

Their diets include small crustaceans, plankton, and algae. However, a few live in fresh and brackish waters, such as the freshwater damselfish, or in warm subtropical climates, such as the large orange Garibaldi, which inhabits the coast of southern California and the Pacific Mexican coast.

Do damsels eat seaweed?

Your damsel doesn't really need the seaweed. It may eat it, especially if it sees other fish eating it. Alone with no examples to follow it may not try it. So basically, yes, your fish is normal.