This icon indicates your currently selected target. If found and scanned, this target will appear on the HUD with this icon, and if you know where the target is through intel, their approximate position will be displayed on the map.
Really the only differences is the somewhat more simplified Nemesis system. Basically, the system is more limited on the PS3/Xbox 360 versions, because it apparently relies on some quite heavy content, calculations and AI.
I have not been able to find a simple comparison of what's missing or simplified. However, it should come down to the system feeling a little more stale, with less variety and less complicated events unfolding.
As quoted from an interview with the design director Michael de Plater:
What it won't have is the same level of depth and variety and simulation within the 'Nemesis system'.
"The story will be the same and the core gameplay will be the same, but [the 'Nemesis system' is] just so huge in terms of content, calculations and AI we'll just have to try and get as much of it in as we can."
Source: http://kotaku.com/shadow-of-mordor-is-way-less-fun-on-old-consoles-1660387779
I haven't played the PS3 version myself, but I can hardly imagine it would be so bad to skip buying the game. However, it does seem that the (few) game critic sites that have reviewed the PS3/X360 versions specifically, do consider it quite badly. Take a look at this review, for example.
Best Answer
The White Skull can be considered the generic one because it just indicates that the Uruk in question is a Captain
The Blue Skull is a Branded Uruk Captain, branded by you ofcourse.
In Battle you can have Yellow Skulls and Green Skulls. Yellow is when you are detected and that Uruk captain is investigating and Green is when he can be finished off in battle.
There is a variety of all the Skulls with a Red indicator, as you can see here:
In the picture you can see a White Skull with the indicator but it can be any type of skull.