Cool, I'll give it a go. what do you think they would grow into? And will I need to feed them? They just seem self sufficient at the moment.
What you're seeing (the medusae) is the adult stage - one of two for a lot of hydroids, including Cladonema species.
These should eventually spawn, and the fertilized eggs will turn into planula larvae which will settle on the substrate and turn into a colony of hydroid polyps (like little, twiggy corals with a sting); once those reach maturity, you'll see them starting to form ephyrae (baby medusa stage hydroids), which will break off from the colony and start swimming around; these ephyrae grow into the medusae, eventually spawn, and start the life cycle again.
For feeding, I don't believe these are photosynthetic, so they'll need zooplankton - artemia nauplii (baby brine shrimp), copepods, rotifers, etc. are all some examples - and/or a tiny food that they can grab out of the water column.
I haven't looked into the dietary requirements of these specifically, so I don't know what size of feed they would need, but I would assume something small (TDO Chroma Boost comes in a good range of sizes, and I imagine they'd have some that would work) - if you're not feeding live feed, then you will likely want to siphon the excess food out or add a CUC that can live with the hydroids to keep the water quality in check (I'd imagine bristleworms would work, but I don't know for sure).