It looks like some difference in rendering on Mono so I guess it's a Mono bug.
On my contact form on Windows it renders like this:
<span id="ctl01_mainContent_ctl01_captcha52" style="display:inline-block;color:Red;height:50px;width:180px;display:none;">Incorrect, try again</span>
but on your site running on Mono the styles are reversed like this:
<span id="ctl00_mainContent_ctl00_captcha5" style="display:none;display:inline-block;color:Red;height:50px;width:180px;">Incorrect, try again</span>
since display:inline-block; comes after display:none; it does display on Mono whereas on windows the display:none; comes after display:inline-block; so it does not display the text until validation fails and it changes the style.
You could try using one of the other captchas. You can use ReCaptha (requires an account) or the Simple Math Captcha. Yiou can choose it from Administration > Site Settings > Security > Anti Spam.
Best,
Joe