Hello, I am not sure if AMPAK(AP) etches, prints or stamps the info on their WiFi modules. If it is printed or stamped, I would guess the smudge is ink from the printer or stamp. It looks like the same color as the info, and if ink, it explains why water did not remove it. Solvents might remove it, however, solvents might also damage the module or surrounding surfaces. Since it is working fine, if it was mine, I would just let it be.