Having been paroled, Alexander was back in Union hands, but he was not allowed to perform any military service under his parole until an official prisoner exchange had been negotiated. So he was sent to College Green Barracks at Annapolis, Maryland, one of several of the way stations for parolees run by the Union. In his pension, Alexander noted that he had an "attack of fever" at Camp Green Barracks but provided little detail.
http://wc.rootsweb.ancestry.com/cgi-bin/igm.cgi?op=GET&db=phillip_walker&id=I09377

Actually, St. John's College at Annapolis, Md.

College Green and Liberty Tree (ca. 1960), St. John's College (MD)
Institution Name: St. John's College (MD)
Original/Historic Place Name: Front Campus, known as "College Green Barracks" during Civil War
Location on Campus: front, running along College Ave.
Date(s) of Construction and Designer(s):
n.d. natural landscape Unknown
Type of Place: Landscape site
Type of landscape–
Large-scale features:
Once a cow pasture, now a more formal campus green.
Other characteristics:
Open space between main college row and residential district of town; used during the Civil War as an area for temporary barracks for Northern troops and as a hospital and parole camp,"The College Geen Barracks."
http://hcap.artstor.org/cgi-bin/library?a=d&d=p1639

Here's more from a Bing search:
http://www.bing.com/search?q=%22College+Green+Barracks%22&go=&qs=n&form=QBLH&pq=%22college+green+barracks%22&sc=1-24&sp=-1&sk=