'Cause' is here always meant in an internal sense : at the subontic level (that is a level where the 'things' are not yet (genuine) things). It is an equivocal use of the term 'cause' (intrinsic cause).
What something is 'essentially', means what somthing is in itself. With respect to properties, though, we must answer the what-question in terms of the cause, because properties have an incomplete essence.