sea lettuce

US /siː ˈlet.ɪs/
UK /siː ˈlet.ɪs/
"sea lettuce" picture
1.

a type of edible green seaweed with broad, flat, wavy leaves, resembling lettuce

:
We collected some fresh sea lettuce from the rocks at low tide.
Sea lettuce is often used in salads and soups in coastal regions.