Although we can't clearly see enough symptoms or parts of the plant to be sure, this sounds like the plant has a scale infestation, which is a relatively common pest for citrus grown indoors. You can inspect the leaves (both upper and lower sides, especially near the center vein) and stems for brown soft scale, using the information on our Scale Insects on Indoor Plants page. Links within that page lead to our general scale information in case you're unfamiliar with these kinds of insects. Management options are also presented at the bottom of that page.

If you do not see scale, feel free to send us additional images for assessment. Pictures of each side of the leaf would help, along with any other suspicious symptoms you may see. A few other sap-sucking insect pests create honeydew, which is the sticky sugar-water secretion that's accumulating on the plant's leaves. Was the citrus located beneath another plant when it developed this residue? If so, maybe that taller plant is the one with scale or some other pest.

Miri