 Solution

Let $v={\mathrm{prox}}_J(u)$ and $p=u-v$. The proximal operator optimality condition gives $p\in \partial J(v)$. By subgradient inversion under convex conjugacy, $v\in \partial J^{\ast }(p)$. Since $v=u-p$, this is exactly the optimality condition defining $p={\mathrm{prox}}_{J^{\ast }}(u)$. Uniqueness of both Hilbert proximal minimizers proves Moreau decomposition:Both terms belong to the same Hilbert space after dual identification. No orthogonality of the two terms is asserted for a general convex $J$; that stronger property pertains to special indicator/cone cases.