Just for clarification, Seagrass is not at issue; it's native vegetation typical of the seabeds adajcent the beaches and part of the natural ecosystem. Yes it does wash up on a regular basis. Some folks have cleared the beds off thier beaches to provide a more "desirable" seaside experience, but most have not.
This year there has been a large amount of sargassum weed which has been driven into the coast line by winds and currents from areas far to the east of Belize. This is unusual, and it has been present in unusual amounts. Dianes comments above well represent the situation at this point.