Is it okay to give plants (specifically, silver leaf maple saplings) expired fish food?

No. The best food for a tree is actually decomposing shavings of its own branches, they self-fertilize. Also, trees attract birds and bird poop is very high in nitrates, again trees take care of themselves. The only care taking is becareful digging near a tree, you can cut its roots, putting nails in it, or weed eating too close where you nick the outer bark, best to put a dirt mound around base, and get multch.

Sure.