I believe feather caulerpa secretes a chemical that is supposed to inhibit Ich.

The idea behind a refugium is that macro algaes will compete with undesirable algaes for nutrients in the water. These nutrients are primarily nitrates, ammonia, phosphates, and dissolved organic carbon. The excess macro algaes harvested from the refugium also provide a source of nutrition for herbivorous fish such as tangs. In addition to all these benefits, the fuge will provide a safe haven for the breeding of desirable invertebrates such as copepods and amphipods.