Plate corals have a tendency to rip their flesh when they are in too high of flow. Do if this happened then it would still need time to heal before re-opening.
I have kept short tentacle fungia for many years. They go through periods when they don't extend and times when they swell up ~10X both body and tentacles and float across the bottom and land on other corals and eat them. Leave it alone and let it acclimate to the spot where it is sitting.
Also, do you keep other corals and are those happy also? We are presuming your water parameters are acceptable for keeping this coral.
My parameters are good except po4 at 0.3
I have my fungia for 2 months it has been opened since the beginning of the introduction. For my angel fish apart from the soft corals he is nice and does not hit my SPS
Because you can clearly see how the sand seems to be pushed away from the glass by the flow and how the flesh of your heliofungia is fully retracted to the point it is getting stretched/torn be the septa on that side as well, the most likely problem is to strong laminair flow.