Go to World 4-3, Amiss Abyss. There is a Metroid hiding in the background just before the underwater signpost with the green arrow. You have to swim past the signpost, go all the way out of the cave to the open area where the jellyfish are circling, and then backtrack into the cave. You will have to take a hit from the electric seaweed on the way back to the signpost. Once you reach it, a school of fish will swim up in the background, followed by the Metroid.