Answer: The mushroom would be the fruiting body of a fungus growing in or on organic matter in the potting mix. You can remove it -- the mushroom and other visible growth -- by scraping it off. Once the material is exhausted, that particular fungus should also disappear. Mushrooms in the soil do not damage plants, so I do not think this is why your plant is not doing all that well.
Chamaedorea cataractarum can show symptoms of salts buildup in the soil, usually it starts as a browning along the foliage edges then increases. Leaching periodically will help, then use distilled or rain water to water your palm. Also take care not to overfertilize it as salts can result from fertilizers as well.
Other possible causes for browning would include too much direct sun (this palm is shade tolerant outdoors, likes moderate but indirect light indoors), lack of humidity (this is especially possible during the winter heating system when our homes become very dry -- it is good to group it together with other plants, use a humidifier or pebble/water tray to try to humidify the room) and even over or underwatering. This plant likes an evenly moist yet well drained soil -- the roots need ample air as well as moisture. It is possible (and a clue being the mushrooms) that the soil mix is either being over watered, is not fast draining enough or has deteriorated over time and become overly dense. To know if you need to water, test the soil with your finger and then be sure to empty the drainage saucer after allowing excess water to drain out.
Another possible cause of browning can be spider mites, these are tiny insects that cause stippling on the foliage and leave fine webbing on the undersides of the foliage and where foliage joins the stem. They thrive in warm dry environments such as in the house during the winter. These can be reduced by washing the plant with tepid water (maybe with a hand sprayer or in the shower or set it outside on a warm rainy day) periodically, and/or you can try commercially formulated insecticidal soap per the label instructions. Be sure to contact all surfaces of the plant from top to bottom including the undersides of the foliage and the stems whichever you try.
I hope this helps!
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