He is a clay mug just bcs he is better off of it. But if he would have Monaco-like game and results and kept his cly results, he would be called clay specialist. And I know that he is not as good as the other players at the very top of the game on clay, but he is in the TOP10 clay courters today, and surely better than Monaco on any surface.
As for Ferrero, you are mixing frogs with grandmothers (Serbian saying
), Ferrero is declining and well past his peak, and Andy is in his prime and hasn't declined since last year. Overall, at their prime, sure I would rank Ferrero over him as a better claycourter