I'm afraid you can't influence this in the HTML export (as opposed to the LaTeX export which allows adding arbitrary captioning code) because the behaviour is hardcoded in Org's org-html-paragraph:
(if (not (org-string-nw-p raw)) raw (concat "<span class=\"figure-number\">" (format (org-html--translate "Figure %d:" info) (org-export-get-ordinal (org-element-map paragraph 'link 'identity info t) info nil 'org-html-standalone-image-p)) "</span> " raw))
Therefore, the only alternative I can imagine is using raw HTML:
#+BEGIN_HTML <div class="figure"> <p><img src="https://imgs.xkcd.com/comics/lisp_cycles.png" /></p> <p>Lisp is a language of timeless elegance</p> </div> #+END_HTML
Did you consider handing in a bug report with M-x org-submit-bug-report?