OK, best thing is call a rehabber.
If you can find the nest, put it back. Birds will NOT reject the young because they have been handled.
Mockingbirds have black beaks, not yellow so no, that's not what it is. Could be starling, the wide yellow beak is typical of them.
They are not protected by the migratory bird act, IF it is a starling, so technically keeping it is not illegal. However unless you are prepared for every half hour to hour feedings from dawn to dusk, better to find a rehabber.
PLEASE DO NOT FEED IT WORMS! That is a myth, it will usually infect the bird with a parasite called gape worm and will make them sick and can cause death. If you can't get hold of a rehabber locally, PM me and I'll help you with what and how to feed. AND NO WATER at this point, either in a dish or by syringe - too easy to kill them with aspiration.