If the information on Find A Grave is wrong, there is a link on the memorial to suggest edits. Just as on WikiTree, Find A Grave has volunteer memorial managers that can correct the information. You may have to join to submit an edit, but it is free. Sometimes you are just submitting a link to parents or correct birth dates. So far I think every correction I have requested has been made. You can also put a note on the WikiTree profile of your ancestor stating that the information on Find A Grave is not correct and that a correction has been submitted. You could do this either in the biography section or in the sources section.
In the Biography, it would be something like, "According to Find A Grave, August's father was Frederich, but this is incorrect according to his birth record." Then add the source citation.
In the Sources, you would just cite Find A Grave and put something in parenthesis explaining what is wrong, why it is wrong, and that you have requested a correction.