a fungus growing on the dead organic matter in the potting mix. If you don't like to have them there, remove and discard.

most such.. are a degrading system of ma nature.. usually associated with the breaking down of wood or other degradable organic material ... i see a lot of wood in your mulch ... and its probably working on that ... removing a plant.. is way over the top ... especially if the plant showed no signs of distress due to such ... be nice to know where you are ... its near imperative on ID ... and fall is prime shroom season due to weather ... in a lot of areas .. presume NONE ARE EDIBLE ... unless you are taught by a local expert ... they arent hard to find ... too many bad ones look exactly like good ones ... go ahead and tell the parental units i told you their reaction was way over the top .. see where that gets you .. lol ... besides.. for most of us.. its getting on late fall ... all those veg are going down soon ... so whats the concern ..?? .. when you boil it all down ... unless of course.. you tell us you are south of the equator. and its spring ... ken

It's not really possible to identify from these pictures. You will need to let the culms mature and grow. There are like 1500 types of bamboo and cross species... There is a large culm that is cut, so you are probably looking at a 30-60ft bamboo species there.

Alain, not sulphurea. Maybe O. leucotricha? It would greatly benefit from being repotted into a larger pot and fresher soil, but maybe waiting for spring is good, too.