Agreenish area around a bacterial colony growing on blood agar indicates

The answer is alpha hemolysis. The other types of hemolysis are beta and gamma hemolysis. While beta hemolysis leaves a clear zone in the area around the bacterial growth, gamma hemolysis indicates that there is a lack of hemolysis and thus the agar is unchanged in the vicinity of a bacterial colony.

A greenish area around a bacterial colony growth on blood agar indicates an alpha-hemolysis.
Blood agar is an enriched, bacterial growth medium, that encourages the growth of bacteria, such as streptococci, that otherwise wouldn't grow on an ordinary growth media. Hemolysis is the breakdown of red blood cells. The medium is used to detect bacteria that produce enzymes to break apart the blood cells. When alpha hemolysis is present, the agar under the colony is dark and greenish.