floating plants
萍 centers on small plants that float on water, especially duckweed or similar drifting vegetation.
萍 combines 艹 (grass/plant) with 泙, which likely contributes the sound and relates to water, together suggesting a plant that floats on water.
The grass top 艹 sits above 泙, which looks like water with a flat surface. Picture tiny green plants floating on still water: 萍 is floating plants.
For ヘイ, imagine floating plants forming a flat green layer like hay on the water: hay -> ヘイ, and the plants drift on the surface.