Dear Vicki, Sebern Silas McCathern is listed on the 1850 census in Sabine County, LA which is one county south of DeSoto Parish and just a few short miles from Mansfield, LA where extended members of the Chapman family who married into the McCatherns once owned a plantation.
This plantation became a Union military headquarters and hospital in the year 1864 during the Red River Campaign in the Civil War.
There was over 60 slaves at this location according to the 1850 Slave Schedules that mentions the Chapmans.
In 1850 according to the federal census, Sebern is listed as being in the housefold or Jas. Wiliams and is listed under the name of Seaburn McCathern, 18 Years old born in Georgia with his occupation as Laborer.
Mr. Williams is listed as a Blacksmith.
Sebern McCathern later appears in an 1852 marriage record with Rachel Ann Chapman and the couple was married on Christmas Eve of that year in the parish just north of where he previously lived with the Williams as a Laborer in 1850.
It appears from the documentation that he was in the State of LA in the year 1850 at the time of the census and was not in Alabama as you have suggested.