This is a fairly recent (last few weeks) development:
It appears to be a bracket/shelf fungus:
From the link, it appears this has already been at work inside the tree for a while? You can see two trunks coming out of a common root area; there used to be three (the third trunk came down in a wind mmm...8? years ago). There were some of these shelf fungus eruptions on (I think this same trunk) about 5 years back, but not since.
What's the prognosis...anything that can be done to either slow or halt the intenral issues?