This looks like a type of wood decay fungi. You did not mention how close this is to the trunk of the tree.  Sometimes the fungus may occur on the roots and may not be a structural issue. 

If the fungus is on a major structural root, we recommend that you contact a certified arborist. They can give you an onsite diagnosis of the health of the tree. They can look for evidence of root decay and the best way to proceed. Here is how to find a certified arborist.

Marian