I've experienced that too only to me the black shadow stayed just like that but as it was in the corner of my eye I thought it could not be .I do think they can slow down, I've seen that too and I've seen them not being able to trespass, I've seen short versions of them and adult versions. It would increase overall but go forth and back too these experiences that I did not even wish to call experiences. Then one day someone came to my home and began asking me if I had had this and the pretty short amount of time that someone was visiting it was starting to effect this person and that was why finally this person said something to me about it. From my understanding there are different beliefs of what these shadows are. I have been told there has been spirit and spirits around me, I was not able to see them myself, not when awake, but in my sleep, dream.

Considering that you can actually see this man is really something but I am also thinking it may not be him that is the shadow at the corner of your eye? If you are that open you might attract them to you. Do you meditate? That way they can see you as a glow and you might bring them back with you, just to use an example.

I would contact someone that can help you with this, to come to your home perhaps and take it from there.