White fungus

I could not identify that exact mushroom by this photo alone, but it does appear to be a white rot. Those will slowly degrade the structural integrity of the tree by breaking down lignin and cellulose, and may ultimately lead to it needing to be cut down. If you wish to have the structural integrity of the tree evaluated, you can call a local certified arborist, or if you wish to get the mushroom identified properly down to its species, you may bring it in to your local extension office, and we can send it off to campus for a fee of $15.