Why is my ex mad I moved on when he dumped me?

Not enough detail, for an accurate answer.

If you “moved on” ostentatiously, so as to more or less flaunt your replacement for him, or to show him and everyone else the two of you know, that you think so little of him that he could be replaced by literally anyone, any ex would logically get angry at the intentional insult.

How fast, and with who, and how did you do your “moving on?”

Why do Trump supporters believe Trump should deport the immigrants? These people you call "illegal immigrants" have lived here for many years, they have houses, jobs, how can you think they will just go back to their country, where they have nothing?

I see the leading responses calling your ex various rude names or accusing him of childish immaturity, but I’ve seen enough different people in the world to know better than to jump to snotty conclusions, especially without evidence.

If he’s in pain that the two of you broke up, which most grown-up people are, and you appear to be unaffected by the breakup at all, again, your “moving on comfortably” would imply that he was meaningless to you, and again, anger would be logical, albeit unfortunate.

If you moved fast, it’s logical for an ex to suspect that the new person was already “involved” with you, while you claimed to be loyal.

Is a narcissist capable of understanding the damage and the hurt that they have caused in your relationship?

If you “moved on” with someone who was a mutual supposed friend, your ex is likely to feel betrayed on two sides of his life.