Green seaweeds are mostly found near the surface of the sea. They can’t use the light in deeper water.
The most common green seaweed you will find around rockpools is the sea lettuce. They have very thin fronds that look like small lettuce leaves. When they are washed up onto the beach and die they turn white and can be mistaken for tissue paper.
Many of the other green seaweeds are not as bright green and can have a yellowish or brownish look. But all green seaweeds are like flowering plants and only use green pigment on its own to make food from sunlight.