Hints: Recall
\begin{align*}
\binom{n}{k}^{-1}=(n+1)\int_{0}^1z^k(1-z)^{n-k}\,dz\tag{1.1}
\end{align*}
and consider
\begin{align*}
\frac{\left( m - 1 \right)!}{\left( m + y \right)!}=\frac{1}{(y+1)!}\binom{m+y}{m-1}^{-1}\tag{1.2}
\end{align*}
We also need
\begin{align*}
\sum_{m=1}^nz^{m-1}=\frac{1-z^n}{1-z}\qquad\text{and}\qquad\sum_{n=1}^{\infty}\frac{x^n}{n!}=e^x-1\tag{1.3}
\end{align*}
Added [2021-10-26]: Here is according to a comment from OP a rather detailed derivation which uses (1.1) to (1.3). It's admittedly not the shortest derivation, but here it is:
We obtain
\begin{align*}
\color{blue}{\sum_{n=1}^\infty}&\color{blue}{\frac{x^n}{n!}\sum_{m=1}^n\frac{(m-1)!}{(m+y)!}}\\
&=\frac{1}{(y+1)!}\sum_{n=1}^\infty\frac{x^n}{n!}\sum_{m=1}^n\binom{m+y}{m-1}^{-1}\tag{2.1}\\
&=\frac{1}{(y+1)!}\sum_{n=1}^\infty\frac{x^n}{n!}\sum_{m=1}^n(m+y+1)\int_{0}^1z^{m-1}(1-z)^{y+1}\,dz\tag{2.2}\\
&=\frac{1}{(y+1)!}\int_{0}^1(1-z)^{y+1}\sum_{n=1}^\infty\frac{x^n}{n!}\sum_{m=1}^n(m+y+1)z^{m-1}\,dz\tag{2.3}\\
&=\frac{1}{(y+1)!}\int_{0}^1(1-z)^{y+1}\\
&\qquad\cdot\sum_{n=1}^\infty\frac{x^n}{n!}
\left(\frac{1-z^n}{(1-z)^2}-\frac{nz^n}{1-z}+(y+1)\frac{1-z^n}{1-z}\right)\,dz\tag{2.4}\\
&=\frac{1}{(y+1)!}\int_{0}^1(1-z)^{y+1}\\
&\qquad\cdot\left(\frac{e^x-e^{xz}}{(1-z)^2}-\frac{xze^{xz}}{1-z}+(y+1)\frac{e^x-e^{xz}}{1-z}\right)\,dz\tag{2.5}\\
&=\frac{e^x}{(y+1)!}\int_{0}^1(1-z)^{y-1}\left(1-e^{x(z-1)}\right)\,dz\\
&\qquad-\frac{xe^x}{(y+1)!}\int_{0}^1(1-z)^{y}ze^{x(z-1)}\,dz\\
&\qquad+\frac{e^x}{y!}\int_{0}^1(1-z)^{y}\left(1-e^{x(z-1)}\right)\,dz\tag{2.6}\\
&=\frac{e^x}{\Gamma(y+2)}\left(\frac{1}{y}-\frac{\Gamma(y)-\Gamma(y,x)}{x^y}\right)\\
&\qquad+\frac{xe^x}{\Gamma(y+2)}\left(\frac{\Gamma(y+2)-\Gamma(y+2,x)}{x^{y+2}}\right)\\
&\qquad-\frac{xe^x}{\Gamma(y+2)}\left(\frac{\Gamma(y+1)-\Gamma(y+1,x)}{x^{y+1}}\right)\\
&\qquad+\frac{e^x}{\Gamma(y+1)}\left(\frac{1}{y+1}-\frac{\Gamma(y+1)-\Gamma(y+1,x)}{x^{y+1}}\right)\tag{2.7}\\
&=\frac{e^x}{y\Gamma(y+1)}-\frac{e^x}{yx^y}-\frac{1}{y\Gamma(y+1)}+\frac{e^x\Gamma(y+1,x)}{yx^y\Gamma(y+1)}\tag{2.8}\\
&\,\,\color{blue}{=\frac{e^x-1}{y\Gamma(y+1)}+\frac{e^x}{yx^y}\left(\frac{\Gamma(y+1,x)}{\Gamma(y+1)}-1\right)}\tag{2.9}
\end{align*}
Cross-check:
We perform a cross-check with the Mathematica generated result stated in the comment section above by @user64494.
We obtain
\begin{align*}
&\color{blue}{\frac{x^{-y}\left(\left(e^x(y+1)-x-y-1\right)x^y+e^x\left(\Gamma(y+2,x)-\Gamma(y+2)\right)\right)}{y\Gamma(y+2)}}\\
&\qquad=\frac{e^x(y+1)-x-y-1}{y\Gamma(y+2)}+\frac{e^x}{yx^y}\left(\frac{\Gamma(y+2,x)}{\Gamma(y+2)}-1\right)\\
&\qquad=\frac{e^x(y+1)}{y\Gamma(y+2)}-\frac{x}{y\Gamma(y+2)}-\frac{y+1}{y\Gamma(y+2)}\\
&\qquad\qquad+\frac{e^x}{yx^y}\left(\frac{(y+1)\Gamma(y+1,x)+x^{y+1}e^{-x}}{\Gamma(y+2)}-1\right)\tag{3.1}\\
&\qquad=\frac{e^x}{y\Gamma(y+1)}-\frac{x}{y\Gamma(y+2)}-\frac{1}{y\Gamma(y+1)}\\
&\qquad\qquad+\frac{e^x}{yx^y}\frac{\Gamma(y+1,x)}{\Gamma(y+1)}+\frac{x}{y\Gamma(y+2)}-\frac{e^x}{yx^y}\\
&\qquad\color{blue}{=\frac{e^x-1}{y\Gamma(y+1)}+\frac{e^x}{yx^y}\left(\frac{\Gamma(y+1,x)}{\Gamma(y+1)}-1\right)}
\end{align*}
in accordance with the derivation (2.9).
Comment:
In (2.1) we use the binomial identity (1.2).
In (2.2) we use the integral representation (1.1).
In (2.3) we do some rearrangements.
In (2.4) we apply the geometric series expansion addressed in (1.3) in the form
\begin{align*}
\sum_{m=1}^nz^{m-1}=\frac{1-z^n}{(1-z)},
\qquad \sum_{m=1}^nmz^{m-1}=\frac{1-z^n}{(1-z)^2}-\frac{nz^n}{1-z}\\
\end{align*}
In (2.5) we use the exponential series expansion addressed in (1.3).
In (2.6) we separate the terms and factor out $e^x$.
In (2.7) we evaluate the integrals (with some help of Wolfram Alpha) using the Gamma function $\Gamma(y)$ and the Incomplete Gamma function $\Gamma(y,x)$.
In (2.8) we collect terms and simplify using the recurrence relations
\begin{align*}
\Gamma(y+1)=y\Gamma(y), \qquad \Gamma(y+1,x)=y\Gamma(y,x)+\frac{x^y}{e^x}
\end{align*}
In (2.9) we make a final simplification.
In (3.1) we use again the recurrence relation given in (2.8).
Sum[x^n/n!*(m - 1)!/(m + y)!, {n, 1, Infinity}, {m, 1, n}] // FullSimplify
which results in $$\frac{x^{-y} \left(\left(e^x (y+1)-x-y-1\right) x^y+e^x (\Gamma (y+2,x)-\Gamma (y+2))\right)}{y \Gamma (y+2)} .$$ $\endgroup$