Please tell me, are benign tumors malignant when they are growing?

Yes and no: Benign tumors typically do not change much(grow very slowly). Any tumor that is growing can be a cancer. So you should get a medical examination to find out what you have. A biopsy is required to find out if it is benign or a cancerous tumor.

Benign tumors: Benign tumors can and do grow, and some benign tumors have potential to become malignant. Just because a benign tumor is growing doesn't mean it is malignant. However, best to see your doctor to figure out whether a tumor is benign or malignant.