I received some great guesses about the identity of these small organisms — and I need to congratulate Marni for submitting a correct answer!
These are indeed planula larvae, in this case from a hydroid called Abietinaria.
Hydroids are colonial cnidarians (related to jellyfish, sea anemones, and corals) with many connected polyps. Colonies often have a branching structure, and because of this they are often mistaken for plants.
Most of the polyps in the colony are feeding polyps — they can extend a ring of stinging tentacles to capture small animals from the water. Other polyps are reproductive and produce the gametes that will form the next generation.
In this case, we were fortunate to have collected a female colony. Within small capsules (gonozooids), there are attached medusoids that produce eggs which are retained and, once fertilized, develop into planula larvae. When they're ready to disperse, the planula larvae emerge from the gonozooids and swim to a new location to begin another colony.
And two more views so you can really see the individual larvae:
Intriguingly, I might have also found a male colony. I'm not sure about this, but it's possible the white, flat-topped structures in the photo below contain male medusoids that will release sperm.
This picture is also helpful because you can see some of the tiny feeding polyps arranged along the branches. I've circled a few in the next image, but I'm sure you'll be able to spot others:
So there you have it! Planula larvae are so tiny that you don't often get to see them, so it's fun to be able to share them with you!
P.S. On 30 January 2014, I showed a fun sequence with a planula larva (from a different hydroid species) emerging from a capsule. To review those pictures, click here.