Apparently he was signed to a minor league deal a month ago.
That we haven't heard anything about him until now is a pretty good indication of his chances of making the squad. Moss is a southpaw who "broke out" in 2002, posting a 3.42 ERA with 12 wins in Atlanta, but it was a mirage - he walked a batter every other inning, and posted the second-lowest BABIP (.232) in baseball. Sure enough, his ERA ballooned to 5.16 in San Francisco and Baltimore the next year, and he spent the majority of 2004 pitching poorly in AAA.
No matter how left-handed a guy is, there's no getting over a K/BB ratio near one. Moss has walked more than 12% of the batters he's faced for his career without establishing a decent strikeout rate, making it virtually impossible for him to succeed. Nobody reading this should hope that we see him in Seattle at any point during the season.