That's a happy looking newt baby.
Yeah I wouldn't say daphnia are super beneficial once they are past the larval stage, but mostly because they're so small. You'll have to feed quite a bit of daphnia to really make much of a difference for adult newts. But I wouldn't say that they're not worth feeding at all though, if you you've got a good culture of them going. Daphnia are highly nutritious and I think could help in giving your newts a balanced diet, even if just as a snack once in a while. Plus it's fun watching my newts chase them around. And this is totally based on my personal opinion, but I feel like feeding your newts these kinds of food that give them a chance to actually exercise their natural hunting instincts/behaviors might make for happier and healthier newts. This is as opposed to feeding exclusively by tweezer or just dropping food right in front of their faces. But again, this is totally just my feelings, and you should prioritize nutrition over anything else.
If you've got a culture of the daphnia going, there isn't any reason you should stop feeding daphnia, at least not during the larval stage.
Frozen bloodworms are generally considered very nutritious as well, given it's from a good brand (Hikari seems to have a good reputation). I have fed my newts those before, but I find that some of them turn their noses up at it if they're also being fed live foods. So, eventually I stopped bothering. But again, you're going to want to feed more thann 1 good food to make sure their diet is balanced. In my opinion, for the larval stage, chopped black worms, daphnia, and bloodworms would be great.
The daphnia will not live long if you're not feeding them. I think really the only good option you have is starting a culture of them. Unless you have a local fish store that sells them or some other local source, you'll be spending loads on shipping buying them online. There's a lot of foods you can feed them, algae (greenwater), yeast, spirulina powder, and various flours (rice/pea/bean). You can culture them in containers as small as 1 gallon, but the bigger the container, the more stable the culture. There's alot of different articles and videos online about how to culture them. Currently I am following the advice from a guy on Youtube called Aquarimax, he's got a few videos on daphnia. I am keeping 2 cultures, one in a 5 gallon tank with some lighting and gentle aeration, and another as a backup culture in a 1 gallon jar with no aeration or lighting. I'm feeding them a mix of spirulina powder and brown rice flour. When I want to feed the daphnia to my newts, I use a turkey baster or pipette to pick some up, and then strain them through a net before putting them in the tank with the newts. If your newt is in a small container, you probably don't want that yeast or spirulina/flour filled water in there ruining the water quality. You also might want to make sure you're using a net with fine enough weave so that it catches the baby daphnia as well, especially if you're feeding to very small newt larvae that just hatched.
Look through some of the info online on different ways to culture them and see what works for you. But I will say for sure you should separate your daphnia into at least 2 cultures, in case 1 crashes. The other thing is people have cultured daphnia with yeast very successfully long term, but it does seem to bring a higher risk of fouling your culture and causing it to crash, if you overfeed. I used to feed with yeast and was never able to keep a culture going for more than maybe 3 or 4 months. Your mileage may vary. Alot of people seem to think that greenwater is the best food to feed daphnia, but I was never able to get a greenwater culture going, so can't comment on that.
After they morph and into their adulthood, make sure to continue giving them some variety in their foods. I feed my adult newts tubifex and earthworms as staples, and am currently trying to introduce more into their diet.
Here's some info on various foods in case you haven't seen it.
I already lost 6 babies
