Well it depends on how she interpreted your question, since it is grammatically incorrect and ambiguous.
You could have meant "can anyone see my signature?" without the "what," in which case you are just asking if anyone can see the green and black text, not necessarily reading it.
You could have meant "can anyone see what my signature says?" with the additional "says," in which case you are asking if anyone can read it.
Maybe the green text is hard to read on a phone without zooming in, but I'm sure it is visible. On the 24" monitor I am at right now with my standard IE zoom factor, it can be seen and read.
Also, it doesn't say "Stop," it says "STOP."