Hi,
It's probably a customization issue.
With recent versions of HikaShop, we've made a new registration page which includes the extraData for plugins to be able to inject their HTML at different places on the page.
If you have customized the view file of the registration form, it's possible that you didn't get the modifications for that.
So try renaming the folder templates/YOUR_TEMPLATE/com_hikashop/user/ and see if that helps.
If it does, it would mean that it indeed comes from one of the view overrides there. IN that case, you should redo your customization on the latest version of the view file concerned.
If it doesn't, then maybe you have an old version of the recaptcha plugin. So download again the install package on our website and install it on yours to get the latest version of the plugin.