"Is mom coming?"
The letter ם is actually the form of the letter מ (the letter Mem - signifies the consonant /m/) used in the ends of words. Therefore, האם (a question word - if you didn't understand when to use האם i'll explain it to you) is spelled the way it's spelled, and not האמ. https://en.wikipedia.org/wiki/Final_form
In a Memrise course I've learned two words for 'mom or mother'
אמא ו אם
One was meant more formal and was pronounced 'eim' (אם) and the other was less formal, something like mom, not mother, and was pronounced 'ima' (אמא). As far as I heard in Israel, all the kids use the second mom-form.