Tightening a Moen Faucet Lock Nut
You could try one of these tools to see if it would get up in there to tighten the faucet anchor nut.



It would be used something like this:



I also wonder about whether you should have first removed the rusty old washer (A) and the dried plumbers putty (B) before installing the new faucet.



First off it seems very likely that the replacement faucet would have come with a new washer. And secondly it would be much easier and secure to tighten the nut against a new washer surface.