This is growing in my potted palm in my house. I just noticed it a few days ago and overnight it grew from about 1" in height to 2" in height. I suspect my cat used the pot as a litter box a few weeks ago when she snuck into this room. Could that be the cause of it? Anyone know what it is? And yes, it is very yellow!
Leucocoprinus birnbaumii . . . one of the regular questions here ;-)
It regularly turns up in plant pots, and is harmless (though I'd not suggest eating them!). It feeds on decaying compost (and maybe helped by cat pee!) in the pot, converting the compost into nutrients that the pot plant can use.