Ghost Pirates of the Sea-erra Madre

You head over to the seagrass to see what the glistening purple shape is. You swim between the seagrass and coming closer to the purple thing you see that it's sitting on the ocean floor half covered in sand. It seems to be releasing a strange glow and is it...moving? That seems weird.

Beyond the seagrass you see what appears to be a cave of some sort.