Modal means "having modes".
In the field of user interface design, a modal interface is one in which a given action has different results depending on what "mode" the system is in. For example, a button on a digital watch might advance the current time by a minute in one mode, and turn on the face light in another mode.
The opposite of modal in this field is modeless. Modelessness is considered desirable in a user interface, because the user doesn't need to think about what mode they are in.
It's not in dictionaries; it's not in my spellchecker. But it is widely used for this purpose. It might work for your purpose too.