When using /fill to replace certain blocks with other blocks, it should replace the block you choose to replace. In this example, what should happen is all regular prismarine is replaced by X block. However, it instead replaces not just prismarine, but prismarine bricks and dark prismarine as well. It does not affect slabs, stairs, and walls of any prismarine variant. This happens when you type the command /fill X1 Y1 Z1 X2 Y2 Z2 BOC(block of choice) 0 replace prismarine. This affects all players on Bedrock Edition that want to replace just prismarine with a certain block, but not prismarine bricks. There is already an option to replace prismarine bricks and dark prismarine, but putting in regular prismarine doesn't work the intended way.