If you take a look at the one by itself to the left you'll see it's covered with minute hairs called cilia. I believe it is a planula larva of a jellyfish that has just settled. I did a search around and couldn't find any pictures of one in this stage. I've seen it myself though. You'll know very soon though because it will grow a stalk that will be attached to the glass and an oval disc. At this stage it could be confused with a hydroid or clear aiptasia.
Or I could be completely wrong and it might be a hydroid developing. Either way let's wait it out a couple days and see?