The main reason while the majority of big aquatic animals are eating other animals, while the majority of the big terrestrial animals are eating plants, is the disparity in size between big animals and most aquatic primary producers, which requires at least one intermediate layer of small animals, who gather efficiently the small algae, packaging the nutrients and energy into bigger bodies, which can be gathered efficiently by the big animals.
The multicellular algae have bodies with a poor content of nutrients, because unlike terrestrial plants they do not need to store large reserves of nutrients in body parts like seeds or roots modified for storage, reserves that would be needed sometime in the future, after finding a place to germinate or after an unfavorable season passes. The stems of multicellular algae typically have only the purpose to anchor them to the bottom, to avoid being swept away by currents, so they do not need to contain much edible substance.
So only a few animals, e.g. sea urchins and some snails or chitons and few fish, have developed the special adaptations required to graze multicellular algae, like adequate teeth, because the reward for such an adaptation is not great and the places with abundant multicellular algae are typically restricted to regions close to shores.