Speaking to / with

"Arm bruise" is understandable and theoretically grammatical, but it sounds a little unusual. In the US you'd more likely hear:

...when I saw the/his bruised arm.

...when I saw (that) his arm was bruised.

A brief look with the google on the intertubes finds that "arm bruise" is a medical condition with the symptom of having a bruise on the arm. It may be caused by anti-coagulant medication, a broken bone in the arm, or falling on your outstretched hands. But you're probably not a doctor, and even if you were, you're not making a diagnosis; you just saw a bruise like any other, which just happened

I was speaking to / with him Both are OK, but in my experience 'speak with' is more a feature of American English.

An alternative to 'the bruise on his arm' is I saw his bruised arm, but this suggests a large bruise or perhaps multiple bruises.