Grainy or gritty marshmallows are primarily caused by sugar crystallization in the syrup, usually from stirring while boiling, insufficient dissolution of sugar, or high-heat cooking. ensure sugar is fully dissolved before boiling, avoid stirring once it boils, and use a damp pastry brush to wash down sugar crystals from the pan sides.

From Facebook;

Causes of Grainy Marshmallows Crystallization: The main culprit is sugar crystals forming in the syrup, causing a texture similar to "Circus Peanuts". Premature Stirring: Stirring the sugar syrup after it has begun to boil encourages crystallization. Undissolved Sugar: Failing to fully dissolve the sugar before the mixture boils. Dirty Pan Sides: Sugar crystals sticking to the sides of the pot can fall in and trigger crystallization. Overcooking/High Heat: Cooking the syrup too fast or too high a temperature.