does look like a nem to me, potentially a great hitch hiker to find but probably not suitable for your tank whatever species it turns out to be
i wouldn't even think about buying a damsel
they get very (read VERY) aggressive and almost always lose their looks as they mature
in a tank your size a fish being picked on has almost no where to hide
two chromis isn't an ideal number, one of the reasons for normally having a group is that it spreads the aggression between the group rather than a single dominant fish bothering the weaker one. personally i would go for a single chromis or 5+