The Arabic word 'Mala" is used for influential chiefs. After seeing these miracles they said to the people he was a great sorcerer. Being ignorant of divine powers of Allah they could say nothing else as they believed Pharaoh to be their god and had seen nothing but the magical charms of the sorcerers in their life. They, however, added the word "alim' signifying the one who knows, showing their impression that the miraculous acts of Musa (علیہ السلام) were of the kind that could not be performed by an ordinary magician. So, they said that he was a sorcerer of great knowledge.
The difference between miracle and sorcery
The miracles and sorcery are so distinct in their nature and effect that they do not require any explanation to any one applying common sense. The sorcerers usually live in impurity and, the more they are unclean and impure the more they are successful in their sorcery. The prophets, on the other hand, are by nature the most clean and pure people. Another obvious distinction is that a sorcerer is never successful when he makes claim to prophethood. Besides, the acts performed under the effect of sorcery do have physical causes as other things have, with the only difference that their causes remain hidden to common people. The people, therefore, take them to be happening without the help of any cause. On the contrary, the miracles are directly a manifestation of Allah's power and have nothing to do with physical causes. This is why the miracles have been ascribed directly to Allah and not to the prophets in the Holy Qur'an. The Qur'an said "but Allah threw the pebbles" (while these pebbles were thrown by the Holy Prophet ﷺ in the battle of Badr) In short, the miracles and sorcery are totally different from each other. The people of knowledge have no confusion about it. In order to eliminate any possible confusion of a common individual, Allah has provided with obvious distinctions between the two.
Even the people of the Pharaoh found the miracles of the prophet Musa (علیہ السلام) somehow different from the normal acts of sorcerers. Therefore, even while accusing him of sorcery they admitted that he was 'of great knowledge' meaning that his act was not comparable with the acts of the normal sorcerers.