Okay, a few things.
First off, I wasnt and am not trying to argue the creature I proposed is viable. It was just meant as an example of a potential way a wheel-like joint analog could work. What I am trying to do is edit it to make it viable. So criticizing what I have without proposing something else is simply unhelpful
Secondly, coral is not a plant. It is a cnidarian, in the same class as jellyfish, and consume plankton in addition to energy from sunlight that comes from their symbiotic algae. If plankton can sustain creatures such as the whale shark, there is not any doubt it could potentially sustain this colony of creature, given that improvements could be made on gathering said plankton. Basically I’m just trying to show the main argument against motile plants is not viable in this case, as this is neither a plant nor something that relies solely on sunlight for sustenance.
Third, bringing up transitional stages/how it evolves being not viable is literally just repeating what I myself have said, once again. I said that was the biggest flaw in my organism, and yet you won’t stop bringing it up as if it is some revolutionary new criticism. I understand it is an issue. I said as much. I asked for assistance in editing it a little/creating a viable step by step process. If you have no input on how to do that, please refrain from repeating things already mentioned.
Finally, yes, I am aware finned organisms is not the only example. I just used it because that is an example of what seahorses would use. Tentacles, Tails, propulsion systems, serpentine movements, etc. are all of course viable alternatives. However, I think it is best for this to assume that for some reason they don’t exist, (as otherwise of course they are simpler to evolve), would wheel like shapes such as the propellor be viable?
Also I noticed you ignored my niche argument, I’m just going to further it a little bit. Things such as cephalopods with tentacles still exist instead of the largely more common tail or fin propulsion, occupying a different niche than most fish. The same could work with this coral, as a mixotroph; I’m not saying it’s necessarily likely, just possible.