I understand it that he referred to him as "that black guy" in Romanian, and so using 'negru' was understood as 'negro' by the Istanbul guys who then assumed he meant it offensively and with all of the negative and racist connotations and undertones that calling somebody a 'negro' in this day and age has with it, hence the reaction.
There are lots of problems when, I'm assuming, none of the Istanbul players speak Romanian and the fourth official wasn't speaking TO the coach in question, rather telling the referee who he was referring to.
I don't think it's as easy as saying "I wouldn't mind somebody calling me that white guy", because clearly white people generally weren't kept as slaves for years and aren't, still, treated differently in society for their skin colour.
I can definitely see both sides of the argument here.
In any case I don't think the official should be telling the referee "it's that black fella there", which it seems is what he said, but I also don't think it was meant in an offensive way at all.