As Ray mentioned, if it is ich you are dealing with, then that is generally brought on by stress or water parameters. Assuming that your water is not the issue (since the parameters look good but good idea to have LFS double check them) another thought on what might be stressful to the fish is that you have two similar looking fish. The yellow tang was there first and will view the lemonpeel as competition because they look sort of similar (color). So the tang would be stressed picking on the lemonpeel and the lemonpeel would be very stressed getting picked upon.

There are lots of good threads to read on this site dealing with ich and your best strategies for treatment. There is also a whole long thread on dosing vitamin c. Good luck!