[Mad Mardigan]

Possible solution.... if you have a save prior to it showing up, on the map... follow through with going to the site of it, scope out if it is indeed that ship & just avoid trying to sink it... as obviously, it is a bugged ship. Waste no ammo on it, and then proceed on with your patrol. This is what I did with a couple that I ran across, that did the same thing that you have ran into here.