Why is there a hard, immobile lump underneath the bruise on my upper left arm? leukemia?

Is called hematoma: Likely you are describing a hematoma- which is blood accumulation on the tissue underneath the skin. When you have trauma that is significant enough to give you noticeable bruise- bleeding process also happens on the area underneath the skin-some blood accumulated plus minus inflammatory process- and combination will give you " a lump". Put some icepacks/cold compress.

Induration: The tissues under a bruise can often be injured and become swollen and hard. We call this induration. I would expect the induration to improve with the bruise. This is not typical for leukemia. I have seen leukemia present with lumps and bruises, but the lumps will grow and the bruises are many. Good luck.