"If you see small mushrooms growing in the soil under or around a bamboo plant, this usually indicates the soil is too moist, either from over-watering or because the plant's soil drains poorly and holds water for long periods. The mushroom that often takes advantage of these conditions is the yellow parasol (Lepiota lutea), a lemon-yellow mushroom that's usually 1 to 3 inches in height. These mushrooms aren't harmful to the plant, but they are toxic to humans and pets, so you should remove them as soon as they appear."

My information comes from here http://homeguides.sfgate.com/bamboo-plant-mushrooms-growing-it-91503.html

I will tend to disagree that it is too moist, in that bamboos vary in what they like to grow in. I have mushrooms in my potted bamboo, this is because I used a compost and this is where mushrooms thrive, as does my bamboo. You just have good conditions for this to happen. If you would remove some of the lower branches from the bamboo, this would allow sunlight to hit the mushrooms, and then in this case, you would see many less mushrooms. Also, mushrooms only need a few inches of good soil to grow, where as bamboo needs a foot or so of good soil. So the mushrooms are only indicators for the very top layer of soil.

I have had mushrooms growing under many of my bamboos over the years and they are not harmful to the plant, they are usually the result of the mulching. I let them alone and in a week or so, they are usually gone.

Kt

Actually, i find that a healthy net of mycelia(the underground component to mushrooms) to be a proper indicator for healthy moisture levels in the ground surrounding bamboo. Once Ive added a good 4 to 6 inches of wood chip mulch over newly planted bamboo, I set up my watering schedule to saturate the plants every week or so. Often I will check the ground while weeding and notice a white layer of mycelia growing through the wood chips. The top layer should appear to dry out in between waterings, if it stays wet then extend the frequency of watering. I turn off the system during rainy seasons to avoid overwatering problems. Once the silica-rich leaves are falling at a high enough rate to create a mulch at the base of the plants, I stop applying wood chips.