rectangle containing text pointing from minipage

Question

I would like to achieve the following:

The current code that I have is:

      \scalebox{0.65}{
\begin{minipage}{0.95\textwidth}
    \begin{algorithm}[H]              %new code
        \begin{algorithmic}
    \caption{Example Algorithm}
    \STATE STEP1 
    \STATE STEP2
    \STATE STEP3
    \STATE STEP4


    \end{algorithmic}
  \end{algorithm}
  \end{minipage}%
   }

and putting the remaining text in another minipage. But it doesn't look like the image I want so please give directions. I am ok If instead of arrow, curly bracket comes

Answer 1

Here I save the individual boxes and then inset one over the other.

\documentclass{report}
\usepackage{stackengine,xcolor,mathtools}
\begin{document}
\setbox0=\hbox{%
  \fboxsep0pt\fbox{\fboxsep3pt\colorbox{black!10}{\parbox{1.5in}{\parskip 1em
  $D = a + b$\par
  $D = a - b$}}}%
}
\savestack{\graybox}{\box0}
\setbox0=\hbox{\fboxsep3pt\fbox{\parbox{2in}{\parskip 1em
  Example Algorithm:\par
  Step 1\par
  Step 2 $D \xrightarrow{\rule{7ex}{0pt}}$\par
  Step 3\par
  Step 4\par~}}%
}%
\savestack\whitebox{\box0}
\stackinset{c}{65pt}{c}{9pt}{\graybox}{\whitebox}
\end{document}

If rotating the pointing arrow is essential, then this small modification could help:

\documentclass{report}
\usepackage{stackengine,xcolor,mathtools,graphicx}
\begin{document}
\setbox0=\hbox{%
  \fboxsep0pt\fbox{\fboxsep3pt\colorbox{black!10}{\parbox{1.5in}{\parskip 1em
  $D = a + b$\par
  $D = a - b$}}}%
}
\savestack{\graybox}{\box0}
\setbox0=\hbox{\fboxsep3pt\fbox{\parbox{2in}{\parskip 1em
  Example Algorithm:\par
  Step 1\par
  Step 2 $D$ \smash{\rotatebox{-10}{$\xrightarrow{\rule{7ex}{0pt}}$}}\par
  Step 3\par
  Step 4\par~}}%
}%
\savestack\whitebox{\box0}
\stackinset{c}{65pt}{c}{6pt}{\graybox}{\whitebox}
\end{document}