Marchaud derivatives allow us to calculate fractional-order derivatives of sufficiently regular but not necessarily differentiable functions. Then, it is natural to ask what kind of continuous functions are annihilated by such derivatives.
I am an admirer of your work on fundamental and inspiring applications and developments of Fractional Calculus. I have know the very very Book of Samko and its papers, but such a question is not answer in this such good references, at least in those I have read. It is needed some advanced mathematical analysis, I have a manuscript on it, but I am not sure that such question is or not an open problem.
I would say that the question is an open problem. If the question is considered in the scale of Lp-spaces, then it follows from the identity (see Theorem 6.1 in Samko-Kilbas-Marichev, as you obviously know)