The history of this building is definitely more interesting than the exterior design. This was built in the late 1960s (cornerstone 1966) for Congregation Mt. Sinai. They had purchased the former Odd Fellows Hall next door at 301-309 Schermerhorn Street and added this annex. They stayed until the 1980s and the Odd Fellows Hall is now a Hare Krishna Temple, while the annex seems largely abandoned. The Brownstoner blog has a good history on the larger building in its Building of the Day column.
The annex is the simplest of buildings with brick face interspersed with what look to be flat columns of concrete or perhaps badly weathered limestone. The front steps are faced in slate. Peering through the front doors, the entry foyer has a window or door on the other side that looks out into a small courtyard space. According to a listing in The New York Times (13 Oct. 1956) the architect may have been Samuel Juster, a minor regional architect with his own Wikipedia page. This seems likely as among his other building are community buildings for Shaare Torah in Brooklyn, at least one of which shares the same restrained design of brick with concrete details. Since both buildings on Schermerhorn are being marketed as a development site and given the changes happening on all sides along this stretch in downtown Brooklyn, these buildings will be gone soon.