the high sea grass does not appear on the menu

Tall seagrass is a block, not available as an item. Its similar to how

1. You can /setblock ~ ~ ~ carrots but can't /give @p carrots

2. You can /setblock ~ ~ ~ potatoes but can't give @p potatoes

3. You can /setblock ~ ~ ~ attached_melon_stem but can't /give @p attached_melon_stem