For those who need the recap, Mosley was introduced to the team during Season 9, after Owen Granger’s death. She replaced him on the team, overseeing the OSP.
But it wasn’t long before it seemed Mosley was abusing her power. It’s revealed early on in the season that she has a long-lost son named Derrick who was kidnapped by her ex-lover. Oh, and that ex-lover happens to be infamous arms dealer Spencer Williams.
Spencer fled the U.S. with Mosley’s son, and she hasn’t seen him since. So, she decides to use her new role as an opportunity to lead an investigation into finding her son — off the books, of course. This also means that Mosley is putting the OSP team’s lives at risk for a personal mission.
Mosley also frequently makes unethical decisions, including firing Deeks after a heated confrontation over the mission and promising an enemy she would sell out her co-workers.
Luckily, Mosley manages to find her son, and is reunited with him the last we see her. This is a happy ending for the controversial character, who is gone by the beginning of Season 10.
While her disappearance isn’t immediately addressed, it’s later discovered that saving her son put a target on her back from the Mexican cartels. This forces her and her son to go into hiding, meaning she can no longer lead the OSP team.
Source : https://www.distractify.com/p/what-happened-mosley-ncis-los-angeles