Rainbows are very prone to this affliction. They get a small growth that resembles a goatee on the bottom lip. Normally nothing to worry about. If I get a little lax on my WC
's mine will start do develop this also. Several days of small water changes(20%) should clear it up. Bows need pristine water conditions, and a diet rich in vegetable matter. Try changing their diet, and step the water changes up to at least 50% per week, with gravel vacuuming and your problems will probably disappear.