
Welcome to TRT!!!
Sorry, but this is not very good news at all. You do need to get them eating as soon as possible. You may need to try some of the live brine from the store. It may also help to soak the mysis shrimp in some Selcon and see if they will take it then.
How much flow do you have in the tank? If you have enough to keep the mysis shrimp suspended in the water column for a little while when you dump them in that may help as well. The seahorses may actually think they are swimming even though they are not.
I wish you the best in getting them to eat and am hoping you will be able to keep them going.