all of the above.
Updating the Giant Basket Star.
I've added the Capable Clambering, Regrowth, Rudimentary Vision and Water Breathing traits of its little sibling but haven't given it Hold Breath as I didn't see any references to basketstars moving around on beaches or rocks above the shoreline like seastars or brittlestars.
For the "disguise themselves as a mass of vegetation or a 'seaweed tree' " I was thinking:
Plant Disguise. A giant basket star can stay in one place and pose its many-branched limbs to resemble a large mass of aquatic vegetation swaying in the water currents. It requires a DC 13 Wisdom (Perception) check for a creature to realize the gorgonocephalid is not a treelike growth of seaweed. If the basket star is concealed within a kelp forest or similar flora, observers have disadvantage on this Wisdom (Perception) check.
Does that seem OK to you?
I like the idea of them using kelp forests for camouflage. That doesn't appear to be a habitat favored by realworld basket stars who like to live on rocks or reefs with open water flowing past them, since they are filter feeders who comb microscopic food particles out of the water passing them. Living in kelp beds is more a brittle star thing.
However, I imagine a giant basket star feeds off way larger food items than the microscopic scraps eaten by their normal-sized cousins, so it (a) favours areas with lots of small fish and the like swimming around and (b) it needs to trick said small animals into thinking its harmless and swim within reach of its arm net. Chasing down a tiny fish would likely burn more energy than it'd gain from eating it. A giant basket star probably only actively pursues meals that are large and slow-moving or immobile (such as dead or injured fish and PCs wandering around underwater!).