Yes it sounds like the Residence Certificate -
ΒΕΒΑΙΩΣΗ ΕΓΓΡΑΦΗΣ ΠΟΛΙΤΗ ΚΡΑΤΟΥΣ ΜΕΛΟΥΣ ΤΗΣ ΕΥΡΩΠΑΪΚΗΣ ΕΝΟΣΗΣ
This is yellow / beige in colour. (The permanent one is blue)
I had to get one last year for work to register me with IKA, again, when I started last season. See also www.livingincrete.net/ResidencePermit.html
If in doubt ask the accounts dept to clarify, in Greek.