I'd say if you're a central midfielder of any standing, Souness is going to have an interest in your game. If you're a central midfielder of standing at the club he hates the most (moreso than Celtic, Scottish folk?), he's going to scrutinise your game and if he finds holes in it, relevant to the talent being spoken about, he's going to dig you out incessantly whilst Liverpool are up and the floor is his, to speak his piece.
Souness is one of the greatest b2b midfielders of all time, this is a fact. Some of the basic edicts doled out from the gods - which Souness lived and died by on the pitch - when it comes to box-to-box play are: workrate, consistency and studiousness, so it is always going to be the bare minimum he expects of any b2b midfielder considered world class.
The plaudits Pogba receives will have him scrutinised even more, so it's a cycle with Souness that will remain perpetual until Pogba gets with the program as Souness sees it.
I don't think it's any kind of 'ism' or anything to do with being foreign or black; I'll put money on Souness having showered praise on Viera, Makelele or Kante, but if you're the creative sort with obscene amounts of talent not putting in the graft he deems prerequisite, he's going to be all over you - the difference between a Pogba and De Bruyne, for example through Souness' eyes will be that one looks like he's giving his all out there and is a sure thing in terms of what can be expected of him in a game, whilst the other is perceived to not be switched on and somewhat indifferent at times, which, to the type of mindset Souness has, is both incomprehensible and unacceptable.
Pogba is his bug bear, not only because he plays for Manchester United, but also for being arguably the most talented central midfielder on the planet who, to Souness, is still not meeting the bar by not being all the grafting adjectives you can usually ascribe to any of the truly great central midfielders of all time.
I'm pretty sure Souness will have no issues with praising Pogba if, to him, he ever turns those elements around.