I saw this sim aquarium with sea anemone, that you can interact with the, they don’t look to be rendered on a flat plain. Using particles would be expensive and slow animation down to a crawl, at least I think so. How were they done? I would like to try it if possible.
It’s most likely either a vertex shader or rigid body joint constraints to allow interaction with player.
90% sure it would be a vertex shader though, do you have images/footage of what you want to replicate?
If there is only a few strands (less than 25) then you should be able to do this with rigid body physics, look in using rigid body join + rigs together for realtime deformation.