Make sure the water quality is good, and have lots of bright light, not just blue light.
Corals need a full range of colour spectrums from light and they need clean water. If there is any ammonia, nitrite or nitrate, they will stay retracted.
If there is insufficient light, they won't expand.
Lack of water movement or too much water movement can cause them to retract.
If the coral was newly imported, it might still be suffering from stress and might need a few more days but normally it's a water quality or lack of light issue.
Looks like a toadstool leather (not a mushroom, which are closer to anemones). Leather corals commonly sulk for a week or more and often shed their slime layer when moved, during which the coral will remain retracted and you may see stuff sloughing off in the current. Large WCs are just as likely to prompt a leather to sulk as to make them perk up. Give it time.