Mindfulness in Nature and Ecotherapy are closely linked. I regard the key difference as being one of guidance. Mindfulness in Nature can be undertaken by anyone, anywhere, anytime - yes even in a city at night! It is a matter of being awake to what is going on around you, being aware of yourself and your connection to the bigger world that we are all part of. Ecotherapy can be a way of developing this mindfulness, certainly the ecotherapist would be encouraging you to become mindful in (and of) nature. The ecotherapist shouldn't be using the natural environment as simply a 'bigger office' but should be highlighting the therapeutic effects of engaging with and supporting nature. There again who could fault anyone for wanting a 'bigger  office' like the one we're using above (Harter Fell in the Duddon Valley, Cumbria)