[Plugin: WP-Members] Can this plugin have an alternative to Re-Captcha ?

There are some hooks in the plugin that would allow that to be customized, depending on your level of php knowledge.

You could use the wpmem_register_form filter to add in your captcha
(from the online users guide: https://rocketgeek.com/plugins/wp-members/users-guide/filter-hooks/wpmem_register_form/)

Then the wpmem_msg_dialog filter could be used to display any error messages. (from the online users guide: https://rocketgeek.com/plugins/wp-members/users-guide/filter-hooks/wpmem_msg_dialog/)