In Arabic, we have a phrase that literally translates as:
To seek gain (make a living/money or profit) on my nape
And we use it when we want to say someon's way of seeking gain is harmful to us as if that person was walking on our nape. For example, when you work for a company and you are caught directing the company's customers to another company that you have links with.
I could imagine that such a phrase does exist with the word "ass". It is close to "mooch off of someone" but it is not quite the same. Another example is when a vendor decides to do business in front of a shop causing disturbance at the shop's entrance. The shop owner may walk up to him and say, I imagine, "are you trying to ... off of my ass?".