Sharks seeming to be able to digest some celulose from seagrasses is not enough to classify them as omnivores, (cats eat grass sometimes but are still hypercarnivores) but is very strange in a fish, because fishes are notoriously bad using sugars. They use fat as main source of energy. Cats have similar problems and will be normally uninterested in sugar.
Regularly consuming plant material for nutrition seems like enough of a bar to render the shark an omnivore.
Not if forced artificially to do so. Keeping its health after consuming plant material would be a necessary step.
There is a reason for giving a shark a diet composed in a 90% of plants and is the same for having vegan dog food. Is a lot cheaper. As animal protein is expensive, we are in a rush for feeding captive carnivores as many plant protein as possible without killing them. You need to tag the animal as "but is an omnivore so this is fine" first.
I had keep a lot of fishes and know when a fish is starving or not having enough food. They turn like tadpoles: big head in a small skinny body. The fish shown in the photo has this symptoms. Either is a very young fish or is sacrifying its own muscle just to survive.