The 18th. had previously served in Benjamin G. Humphreys' Brigade. This site suggests the 18th. was still in Humphreys' Brigade in the last days of the war.
I did find three other members of the 18th. in the Point Lookout prison roll, these men were also captured April 6 but High Bridge was listed as the place of capture.
These three men along with Moses Pearson were received at Point Lookout on April 14 from City Point, VA.
According to some family researchers, Moses Pearson's wife was Rachel McLaurin. Quite a few McLaurins from MS had ties with Marlboro County, SC. These men may be Pearson's connections in this unit.
(Company A was raised in Rankin County and Company K was raised in Hinds County.)
Private A. McLaurin - K
Private Daniel A. McLaurin - K
Private Duncan McLaurin - A
3rd. Lieut. Hugh D. McLaurin - K
Sgt. John McLaurin - A
Corp. R. Lewis McLaurin - A
Corp/Pvt John W. Pearson - A