Oyster is one of the elements or collectibles on Farm Heroes Saga.
On a regular farm, you wouldn't expect to see any oysters. Farm Heroes, however, puts oysters next to carrots. Oysters need to be opened by 2 sequential matches next to them, in which the 2nd match can be an auto-match. They will close again if the 2nd match doesn't happen.
This item is movable with cropsies.
It is also a feature in Pearl Paradise event.