AMC hasn't confirmed the casting or revealed who he'll be playing, but he's probably playing Abbud, a character for whom a casting call went out earlier this year.
"Abbud, an innately likable Muslim-American whose nerves are, let's say, jangled, because he's flown solo for too long in zombieland," the character description read.
Comicbook.com suspects that Abbud is based on a character from the comics named Siddiq (or may even be Siddiq, considering casting calls often use fake character names), who was a survivor from the Oceanside colony. The character would necessarily be different on the show, since Oceanside was introduced earlier than its comics counterpart and is inhabited exclusively by women because the Saviors massacred all the male residents. In the comics, he lives at Alexandria and becomes romantically involved with Rosita.
Perhaps Abbud is a male Oceanside resident who survived/fled before the massacre and has been on his own ever since. Seeing him reunite with his loved ones could be an interesting Season 8 subplot, especially if he fled due to cowardice.
Nash can briefly be seen in the Season 8 Comic-Con trailer.
This would be Nash's most prominent TV role in his career thus far. He previously appeared in a Season 2 episode of Silicon Valley as the founder of "Bro 2 Bro," an app that texts the word "bro" to the user's bros.
The Walking Dead Season 8 premieres Sunday, Oct. 22 at 9/8c on AMC.