What is the name of an impression formed when tiny plants or animals become trapped between sediment layers?

either trace fossils or mold fossils or both since both would fit in your description but its probably mold fossils if you dont think so here is a list from enchanted learning

mold fossils (a fossilized impression made in the substrate - a negative image of the organism)

cast fossils (formed when a mold is filled in)

trace fossils = ichnofossils (fossilized nests, gastroliths, burrows, footprints, etc.)

true form fossils (fossils of the actual animal or animal part).