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There is a painting on the back wall reflected in the mirror, Luigi must use the Dark-Light Device on the painting releases the yellow "gem". There's a hidden pressure plate near the tub, which reveals a treasure chest filled with gold bars.
After Nikki, Lindsey, & Ginny scrambled the door ways, it is east of the Bladed Bedroom. When Luigi enters the Mirror Bathroom again, he is attacked by three Oozers, all of which must be defeated to proceed.