"Their bed sheet is not clean."
Translation:המצעים שלהם לא נקיים.
It seems like the given translation might be inaccurate. מצעים generally refers to sheets, pillowcases, blankets, pillows etc., and it does not have a singular (you just have to use the name of the specific piece of מצעים you're using). Wikipedia suggests bedding or bedclothes as a translation to English.
As for bed sheet - if I understand correctly it's either the cover of the mattress or a piece of cloth between the blanket and yourself - that would be סדין in singular and סדינים in plural.