its always good to have a backup function for the macro so you can heal, if your spell is heal, if you do not want to mouseover.

example would be....
/cast [target=mouseover, help] [target=player, mod:alt][] Heal

this will first check if you have a friendly player on mouseover. if yes, then it heals, if not, then it goes to the next condition. do you have alt pressed? if yes, it heals yourself. if not, it goes to the next option, which is basically, Heal. so it will attempt to heal your current target.