The size of the larvae makes a big difference to what you can feed them. If this is a type of Lysmata larvae, then newly hatched live brine shrimp are worth trying, but you can try dead zooplankton or other plankton-sized mashed up marine foods if you are willing to do a lot of cleaning out uneaten food that sinks to the bottom when the larvae fail to bump into it or just aren't interested. If you are serious about trying to raise shrimp, have a look at the book "How to Raise & Train Your Peppermint Shrimp" by April Kirkendoll. Shrimp larvae require an incredible amount of time and a special setup to handle effectively. I gave it a try and had neither the spare time nor space required to do it successfully.