Snowflakes growing in my tanks.

[ScubaSteve]

My guess is that they are jellyfish. Jellyfish start out as polyps similar to what you saw on your glass and then release several offspring from a single polyp. If they are free swimming I woouldn't worry too much about your urchins and starfish. I'd be more concerned about your corals/fish. But since these are babies they won't really pack a huge sting, in fact, some of your fish might eat them depending on what you have. Take a picture! I wanna see, I wanna see!