I'm going to assume the party is patient. (more or less)
The walls, ceiling, floor, and bars were shaped by magic, by someone or something with immense power and control over stone.
Nothing in the room, including the gold mask, is magical.
Since there is no obvious way of opening the cage or bars perhaps the owner just shapes them out of the way.
Closer observation also reveals a food/water trough in the stone cage. With a small amount of water in it.
The manta ray gradually becomes accustomed to your presence and resumes floating listlessly.
When Plool slips between the bars blocking the room, The creature startles, and with a flap of its wings brings a gust of fierce wind singing through the room. It presses Plool briefly against the wall, but when it is obvious that Plool is not charging the cage, the wind dies away again.