Charlus Potter: Rowling’s Black Family Tree mentions a Charlus Potter who was married to Dorea Black (1920-1977), a grandchild of Phineas Nigellus Black (BFT). They had one son (unnamed) (BFT).
Charlus Potter is part of the Potter family, but shows up on the Black Family Tree because he married Dorea Black.
It is doubtful that Charlus is Harry's grandfather because Jo has told us that "James's parents were elderly, were getting on a little when he was born, which explains the only child, very pampered, had-him-late-in-life-so-he's-an-extra-treasure, as often happens, I think. They were old in wizarding terms, and they died." (TLC) Dorea was only 57 when she died.