Answer: I can't really diagnose a plant problem without knowing the name of the plant and whether it is outdoors or indoors. Since you mentioned yellow mushrooms, I'll assume the plant is in a container. If so, and if it is planted in potting soil rather than garden soil, the mushrooms are growing in the decomposing organic matter. They need a moist environment in which to grow. They are harmless to your plants and to your potting soil. You can simply pull them out and throw them away. If leaves are falling off your plant and mushrooms are growing in the potting soil, I'd guess that you're keeping the soil too wet. Saturated soils can suffocate plant roots and cause dropping leaves. If you submit another question with the type of plant you're growing and the kind of care you're giving it, we can help you determine what the problem might be.
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