Little hydroids. Basically teeny tiny jellyfish. They're not a great thing to have but are very, very common. There are many types of hydroids so don't freak out when you do a google search. These little guys typically hang out and then just disappear. Some folks call them "soft hydroids" or "snowflake hydroids." Use those search terms and you'll get all the info you can handle. To make it simple, I can sum it up here: They aren't good, usually aren't really bad and there's no reasonable treatment for them. However, I've never known anyone to have an issue with them and lots of people have them.