Is there a difference between microcytic anemia and beta-thalassemia?

Yes: Microcytic anemia means a person is anemic, and part of the reason is that his red blood cells are quite small. Beta thalassemia is just one of the causes of microcytic anemia. Microcytic anemia can also be caused by iron deficiency, Alpha thalassemia, etc...

Thalassemia: Thalassemia is a deficiency in the production of one of the globin chains (alpha or beta) that make up hemoglobin. Normally synthesis of these is balanced. When imbalanced, one globin accumulates relative to the ither forming inclusions that are damaging to the red cells that contain them. The rbcs of a pt with thalassemia are microcytic, but there's other causes of microcytosis. Fe deficiency #1.

Simple term issue: Microcytic = small cells. anemia=low blood volume; there are many causes for microcytic anemia. Beta-thalassemia is a condition where only small cells are made.