I advise you to have a look at Mainardi's book on viscoelasticity or, Mainardi's exclusive articles on Mittag Lefler function (available on Arxiv too, I guess). It is strictly monotonic decreasing, but yet positive as I remember.
I aslo checked the property. There is another simple way to prove. You can consider the corresponding line equation of fractional order and together with the uniquness of solution, you will derive the property by contracdition from the equation.