So, I was browsing the wiki, and clicking on stuff, when I saw one about Nala's father. It says that it's an unknown male lion, and that it's even mentioned in the Lion Guard. So I went back and watched episode 11 of the first season. The lion mentioned IS her father, but Rafiki also says that it's "Kion's grandfather, when he was a cub", so in short it's Mufasa.