How big is she? less than 1.5'' its probably H. zosterae. Hydroids are a type of "jellyfish" to be simple that can free float, swim, or attach themselves to anything. Then can be harmful to H. zosterea b/c they are so small and can steal their food, almost every tank has them. dwarf tanks especially have them because the large amounts of BBS are perfect food for them, so you have to be carful and make sure they arent causing problems. You can get a medication for hydroids or can try freshwater dips. Eleminating them is very difficult, though because they can be present in BBS eggs. Hope this helps.