The cache is NOT located at the posted coordinates. That is the location of Oakland Baptist Church. You do NOT need to go there while on your Adventure Lab journey. You need to complete the "African Americans of Alexandria" Adventure Lab series in order to obtain the real coordinates for this cache.
Local African Americans helped shape Alexandria's history since the early 19th century when free blacks established a community called "The Bottoms" located in the southwestern edge of the city. Up until 1846, when Alexandria merged from the District of Columbia to the Commonwealth of Virginia, the free black population increased continuously. During the American Civil War, hundreds of contrabands (runaway slaves) took refuge in Union-occupied Alexandria. Hardships occurred, however, for contrabands as many refugees caught diseases and died while Alexandria was under Union control. After the war, local freed blacks established a handful of communities in Alexandria, including "The Fort" located in what is now Fort Ward Park. These communities lasted for nearly a century until infrastructure development displaced its residents. African Americans contributed to important local events that took place in Alexandria such as the Alexandria Library sit-in of 1939, one of the first major sit-in actions in the United States, and Jones v. School Board of the City of Alexandria, a 1959 case that resulted in school integration in Alexandria.
The link to the adventure lab caches can be found here. You can also open the Adventure Lab app and find the series under "Explore."
Note: Originally at the time the AL caches were released, they were in linear order, but I had a change of heart. They can now be done in any order you choose. However, I recommend that you start in Old Town and finish at Fort Ward Park as the bonus cache is located on the western side of Alexandria.
The coordinates for the bonus cache are N 38 4A.BCD W 077 ED.EBD