In the movie, you learn that Mal is the daughter of Maleficent (Mistress of evil/ Dark fairy) and Hades (Greek god of the underworld). To answer the question: Is Mal an immortal being? First, you have to look at the facts.
Let’s first look at Maleficent. In the lore, Maleficent is said to be a powerful dark fairy who’s said to get her powers from a Phoenix. People believe that she gets her powers from her scepter, but like Thor (Norse god), the scepter just helps contain her power. It is said that the Phoenix can be almost as powerful as a god and can live up to 500-1,461 years, but since they are immortal, they are reborn in their own ashes. Maleficent has a direct line to the power of the Phoenix that she is considered semi-immortal. She can still die, but it would not be easy to kill her.
Now let’s look at Hades. Hades has been around since the creation of the gods. He is the Greek god of the underworld and is one of the three original gods (Zeus, Poseidon and Hades). Since he is a god, he is immortal and cannot be harmed or killed by a mortal. Only a god can kill another god. That’s basically all there is to talk about besides the fact that he has a helm made for him by the cyclopes that’s capable of making him: invisible, gives him power to cause intense fear which could’ve led to madness, control over the dead (a power he already had), and intangibility.
Mal is the daughter of a semi-immortal dark fairy and an immortal Greek god making her a semi-immortal Demi-god. She is the clear definition of what a Phoenix is. All this leads to one simple answer. Is Mal immortal? Yes and No. Based on all the facts that led us here, she is an immortal being capable of dying from natural causes and murder. But since she has the powers of both a dark fairy who has a direct line to the power of the Phoenix and the power of the god of the underworld, she has the power of rebirth, meaning that if or when she dies, she will rise from the ashes and be reborn once more.
I hope this answered your question.
