It appears Alexander himself was the first to report the news:
Alexander played in nine games for the Seattle Seahawks a year ago but hasn’t started a game since 2017. While he has 23 starts over his five-year NFL career, all of them are at safety while the Bills list him at linebacker. Instead of competing with Rafael Bush, Dean Marlowe, and Siran Neal for a role as a backup to Micah Hyde and Jordan Poyer, it looks like he’ll be in competition with Corey Thompson, Julian Stanford, and Deon Lacey.
He was picked by the then-St. Louis Rams in the fourth round of the 2014 NFL Draft. He has two career interceptions to go with a fumble recovery and 118 tackles. He played 113 special-teams snaps for the Seahawks in 2018, or about 29% of all the special teams plays, but didn’t take a single snap on defense. In 2017 with the Rams, he was in on 21% of the defensive snaps after playing 84% in 2016.