documentclass{beamer}
usepackage[utf8]{inputenc}
usepackage[english]{babel}
usepackage[plain]{algorithm}
usepackage{algorithmic}
usepackage{array}
usepackage{eqparbox}
renewcommandalgorithmiccomment[1]{%
  eqparbox{COMMENT}{#1}%
}

begin{document}
begin{frame}
begin{algorithm}[H]
  begin{algorithmic}[1]
    STATE $x := 1$ visible<2->{COMMENT{$x_1 in [1;1]$}}
    WHILE[visible<2->{COMMENT{whatever}}]{{$x < 10000$}
      STATE $x := x+1$     COMMENT{bla}
      ENDWHILE
  end{algorithmic}
end{algorithm}
end{frame}

end{document}