All acid-fast bacteria are Gram positive - and will therefore retain the purple color of the crystal violet of Gram staining. But the reverse is not true; not all Gram positive organisms are acid-fast.
Yes acid fast bacteria are clear in gram stain samples, and they might be postive and a negative gram stain after testing, depending on the condition of infection of the patient.
Yes, acid fast bacteria can not be stained using this method so the bacteria are basically invisible. These are usually labeled gram negative even though that is not necessarily the case.
