tikz fill[black] (1ex,1ex) circle (1ex);

! Use of use@pgflibrary doesn't match its definition.
pgfutil@ifnextchar ...1def pgfutil@reserved@a {
                                              #2}def pgfutil@reserved@...
l.375 ...blah blah.}

?

documentclass{article}
usepackage{tikz}

begin{document}
tikz fill[orange] (1ex,1ex) circle (1ex);
begin{figure}
caption{tikz fill[orange] (1ex,1ex) circle (1ex);}
end{figure}
end{document}

caption["Short" caption without tikz code]{Long caption with tikz code}

caption{Some caption with tikz code: protecttikz protectfill[black] (1ex,1ex) circle (1ex);}

% At best in the preamble:
DeclareRobustCommandmytikzdot{tikz fill[black] (1ex,1ex) circle (1ex);}

% later
caption{Some caption with tikz code: mytikzdot}