I had to tinker for a while before realizing my mac was confused by the default setting for creating vnc connections on ubuntu. Here's a summary
to enable vnc access to your ubuntu desktop
Settings -> Sharing -> Remote Desktop
then Enable Legacy VNC Protocol. Notice there is a three-vertical-dot submenu. Select "require password". The default, "New connections must ask for access", confuses my mac. It could be a timing issue.
You can then use the native vnc client on mac by going to
Finder -> Go -> Connect to Server
and then entering
vnc://<your server name or address>
