Martin was a second-round pick in the 2009 draft, and the Carolina Panthers tried to convert him to safety -- despite playing cornerback at Troy. His best season came in 2011, where Martin totaled 68 tackles, 8 passes defended, and 3 interceptions while playing in a traditional free safety role.
The Rams selected T.J. McDonald in the third round of the 2013 draft, and the rookie will compete for the starting FS job. The depth chart is full behind him too, meaning Martin would be an outside shot to make the roster, if signed. The Panthers tried to rotate him at both safety spots, but his lack of ability as a tackler made him a liability.
This lack of tackling ability, paired with injury concerns, caused the Panthers to move on. Martin has ball-hawking ability, and his best move could be to transition back to cornerback -- but this seems unlikely following years of training in a safety role. Martin could be a low-risk pickup off an injury, and has the ability to shine in training camp -- provided he can see enough snaps in a crowded secondary.