It should be 걱정거리 to fit the context. 적정거리 doesn't exist. 걱정거리 is made up of 걱정(worry) and 거리(things), to mean something that worries you. 거리 is a dependent noun with a loose meaning of things or source material, work to be done, collection of something, etc. It creates common phrases by combining with other nouns. For example, - 일거리 = work to do. - 얘깃거리 = things/subjects to talk about; gossip material. - 걱정거리, 근심거리, 고민거리 = things that worry you; a concern or worry. - 저녁거리 = stuff to cook for dinner. - 국거리 = ingredients for soup. - 소일거리 = things for pastime. - 논쟁거리 = point/subject of contention.