If you create an HTML page which has an empty <div> tag for insertion of your SeaGreen form, you should not have any problems using SeaGreen with Joomla.

SeaGreen offers a "match field" which mimics the Captcha functionality. You can require that users type in an alpha numeric code which you may provide in text or image format (much like the way Iconico does things in this forum).

Just beware that if you insert actual Captcha code into the Code Editor of SeaGreen, be sure that you save a copy of that custom code in case you edit the form in the Form Design view - this action re-writes the code on the fly and will overwrite any custom code you may have previously entered.