The leaf injury is hard to diagnose from the images, especially since we can't zoom-in due to the smaller file size, but we have a few guesses. What looks like stippling (fine pale specks) could have been caused by spider mites or thrips, both common indoor/tropical plant pests. Ants hunt for insects to prey on when they roam over plants, but certain insects (sap-suckers that produce honeydew, which is a sugar-water waste liquid) they protect instead. When this happens on indoor plants, it's often soft scale insects. We can't tell for sure, but there might be a couple scale insects under the ants in the photo, but you can inspect both the stems and the leaf undersides for indications of scale, based on information in the linked page.

Our Growing Dwarf Citrus page has more information about troubleshooting problems as well as routine care to keep plants as stress-free as possible.

If you find what you suspect are thrips, scale, or mites and would like confirmation, feel free to send us additional photos for feedback.

Miri