Overwatering or a nutrient gap

What's happening

Yellowing across many leaves at once is most often advanced overwatering, so the roots are the first thing to check. Less often it's a Pilea that's been sitting in the same exhausted soil for a year or more, multiplying babies it can no longer feed, and has simply run out of nutrients.

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 the mushy roots, and repot into fresh mix. If the roots are firm and healthy and it just hasn't been fed in ages, resume a balanced liquid fertilizer at half strength every few weeks through spring and summer to bring the green back.

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