Hello! It could be splitting into two separate polyps. Mushroom coral sometimes act strangely during this process. Or it might be digesting a large piece of food. If the other corals and fish in the tank look healthy, then have no worries.
In the meantime, you can test your parameters such as salinity, nitrate, and phosphate to make sure nothing is out of balance.
Beautiful coral, and hope I could help!
Update: mushroom is splitting! I'm so excited to have two of them