Yes, it can be a sin to speak ill of people for many reasons. Sometimes it’s mortal and sometimes venial; however, I don’t think that was the question they were asking. I think they were asking a specific question about whether to confess face to face because the person on the other side of the screen would be part of their confession.
To the OP: Totally my opinion here, but I would treat this in two different ways. If it were me, I would handle my confession as I always do and confess behind the screen. I probably wouldn’t even mention which priest, just say, “I said some disparaging and negative things about a member of the clergy to other people and participated in the sin of gossip and possibly denegrated their character.” After confession and at another time I would personally apologize for the occurance to the priest. I admire your desire to charitably apologize in person. I think it is the right thing to do, but I think you can do it individually and not in the context of your confession.
Just my two cents. I hope it helps.