@jphilip813 asked;
So are they suppose to move?
They can do it; sometimes they do and sometimes they don't in our aquaria. I don't know if they move because they want a better neighborhood, perhaps they are seeking a more food-rich location, or different current exposure, more daylight, or another plate coral to spawn with, or????
I've had mine a few years, one was a tiny green dot attached to a chunk of live rock that was sold to me as a zoa frag. Another my kids got me for a birthday present. That one was the size of a nickel and may have weighed a gram when I got it? When it was picked up for bagging at the lfs, the handler broke the wafer-thin plate skeleton so it didn't lay flat. The skeleton on that one is about the size of a teacup saucer and the injury makes it look like a bent auto rim that got badly out of round from hitting a curb.
So I have observed these two for years from their childhood through adolescence. They go through visible stages of living. Sometimes they will deflate completely so that the skeleton is very visible and they will appear to be on deaths door or even dead. (I rescued a long tentacle plate that came in "dead in the shipping bag." I literally took the "skeleton" out of the trash, rebagged it, and brought it home to my tank. I returned it a week or two later in perfect visual health, bright green & fully extending tentalces and someone else took it home.)
You are feeding it properly. These are made to collect drift food. Mine capture scraps of the fishes food but those morsels often get picked off of the plate by hermit crabs or hungry fish that scavenge by "licking off the plate" after their meal. When they were small I did target fed with frozen or freeze-dried plankton but have not done that for years.
On one reef I have observed a few of them in the wild. Those were gathered on a descending slope inside of the lagoon in pockets where the currents would sweep over them. They were not feeding (it was day time) and they were retracted and less puffy than the plate in your picture.
That puffy look, as others have mentioned is generally taken to mean that the plate is happy & healthy. If yours should ever deflate so that the skin is sucked up against the skeleton don't assume that it is dead. MIne have done this many times and I don't know why? They came back around to puffy, and sometimes they will go super-puffy.
(I didn't finish my camera downloads, I still owe you a picture.)
Superpuffs inflate with water increasing their surface area so much that they approach neutral buoyancy. In this hyperinflated state, they can move (walk?) about on the currents like an inflated beachball in a windstorm. Uninflated they can cling to a steep ledge near the reef crest where they probably inflate slightly during slower tidal flows to capture some sunlight for energy. Otherwise, it seems they can capture enough drift food to live happily in this deflated state with minimal food capture from symbiotic algae inside their bodies. (Not peer reviewed info, just a swag, pers ob.)
Also, if the plate "dies" do not instantly remove the skeleton from the reef (unless it's making a huge rotting mess of the tank). These "dead skeletons," like some other lps corals have been observed to "sprout babies" and you could end up with many clones from the one mother. My nickel sized plate was a sprout, the bag handling damage may have resulted from the bagger poorly snapping the baby plate from its attachment point on the "dead" mom?
