how do I write this in XeTeX?

This is no "weird font": this is Euler script. It can be used for instance with the package eucal, as in the following example.

documentclass[11pt]{article}
usepackage{amsmath}
usepackage{eucal}
begin{document}
( operatorname{Ho} mathcal{C}_{cf} to operatorname{Ho} mathcal{C}_f to operatorname{Ho} mathcal{C} )
end{document}

It can also be loaded with the option cal=euler of the package mathalpha, as in the following example.

documentclass[11pt]{article}
usepackage{amsmath}
usepackage[cal=euler]{mathalpha}
begin{document}
( operatorname{Ho} mathcal{C}_{cf} to operatorname{Ho} mathcal{C}_f to operatorname{Ho} mathcal{C}. )
end{document}

Both examples give the following output.