The species
Macrognathus siamensis is not in SF, but another species
M. zebrinus in the genus is, and I suspect diet would be similar:
In nature, it preys on small fish and aquatic invertebrates but aquarium specimens readily accept dead alternatives. Prawn, cockle, mussel, earthworms and bloodworms can all be offered. Dried foods are rarely accepted. Like others in the genus, it’s quite a shy fish and you may have to add food after lights out for a while until it becomes accustomed to its surroundings.
A caution though, on bloodworms--these are not especially wholesome or nutritious so I would wean any fish off these ASAP. Back in the 1980's one of my prime frozen foods was squid, not sure if this is still available (in packages as fish food, similar to brine shrimp, bloodworms, etc). Also used frozen plankton.