Someone will answer who is more knowledgeable than me but I have similar stuff growing where we had a tree removed. It's some kind of fungus that feeds on the decaying roots. I guess the good news is I've had this issue since last year and it hasn't spread or gotten worse. It's confined to two spots on either side of where the tree was. I sometimes pour boiling water on the white stuff and it blackens and dies back, but then it will reemerge.

Kathy

If you are very lucky it is one of the fungi that decomposes wood. Now that it is started, it cannot be turned off. I would not try to stop it. Instead I would encourage it with the occasional drink of water. I had a stump buried under compost for 10 years. I finally unburied and kept it moist to start the fungal decay process (sometimes called dry rot). Once that stuff started there was no going back. The 10 year old stump has disappeared just 24 months after I really went for the fungal approach. The worst thing you could do at this point is try to bury it under the soil. Let it breathe and keep it moist.

Thank you for the responses. I actually stumbled across the Univ of MD Home & Garden Information Center website where you can submit a question to them and they will respond back. They told me what I have is Ganoderma fungus, which is the 2nd most common wood decay fungus. They said there is nothing that can be done about it, but that it should come and go until the roots below the surface are fully decayed. I'm glad I was able to solve the mystery and happier that it isn't going to take over my entire lawn.

If anyone else is interested, the website for the U of MD was http://www.hgic.umd.edu/