Overwatering or a nutrient gap

What's happening

Yellowing across many leaves at once is most often advanced overwatering on this rot-prone plant, so the roots are the first thing to check. Occasionally it's a Pilea that has sat in the same tired soil for many months without being fed and has simply run out of nutrients to support its fast, leafy growth.

How to fix it

Check the soil and roots first: if they're wet or rotting, treat it as overwatering — dry the plant out, trim away mushy roots, and repot into fresh mix. If the roots are firm and healthy and it just hasn't been fed in a long time, resume a balanced liquid fertilizer at half strength every two to three weeks through spring and summer to bring the color back.

This is general guidance based on common symptoms; individual plants vary.