What was the construction of the tray? is it suitable for smaller mosaic tiles? they look like the ones from topps which are about 40mm or so. A fair few wet room trays aren't suitable for smaller mosaic tiles due to point loading so it may all be a non starter anyway. But the answer to your original question is yes you can remove tiles, slow going, very slow going with a grinder you can cut down to remove material but i wouldn't advise this be done by anyone but a competent tiler.