He was the master of the horse at the Hessian Court.
Here's what The Romanovs has to say about the situation:
(Upon Tsarevitch Alexander falling in love with Marie Hesse) 'Early maturity had owed something, perhaps, to the unhappiness of her childhood; her parents had been estranged for some years before her mother's death from consumption, when Marie was eleven. It was said that not only had she been born several years after her parents ceased to live together, but also that the Grand duke's master of the hosre was her father, and that the Grand Duke had only acknowledged her..in order to avoid a scandal.'