tankless toilet has slow leak from wall/rusted pipe. Is leak in wall? from exposed pipe? what to do?

Answer 1

If you mean flush valve type, it should be a 1 inch nipple into the wall. shut off the water, unscrew the stop, if it's soldered you've got a problem, if it's threaded, matchup the nipple and replace it.

Answer 2

This sounds like more than just a simple job, so my advice, despite being more expensive than a "do-it-yourself" project, is to call in a licensed plumber to take care of it. Good luck.