Nessus Scan Port ID mapped to Metasploit Vulnerability exploits

In terms of finding the corresponding exploits for Nessus output, there's a couple of good techniques.

Firstly, Nessus puts a flag in each report item for "exploitable" which indicates that there is exploit code available.

Once you've got that you can use the CVE number which will likely be listed in the finding (or the MS number for Microsoft vulns) as they're unique to a specific issue, this lets you find the exploit quickly, if it exists in Metasploit.

As to automatic exploitation, such a tool could be written, but it would be a bad idea in most cases as exploits have differing levels of reliability and in a number of cases they risk crashing the host or the service that they're exploiting, so a safer way is to analyze your options once you know what exploits will work on a target host and pick the one with the best Metasploit ranking, which should give you a more reliable exploit.

I just found ExploitSearch. If you search "NESSUS [NESSUS ID]" then it will take you to some details on that nessus id exploit and that contains links to metasploit's website for a module to exploit that vulnerability. I just found it so I'm not sure if this totally answers the questions, but maybe this will point in the right direction.