Eggcrate (white) light diffuser does not cause (depending on your definition I suppose) algae to grow. It does provide an exellent place to grow algae. It's white and reflects a lot of light back ensuring good lighting for algae. It's shape and slightly rough edges give algae a great place to grab on and take "root" and it makes it annoyingly difficult to clean should small amounts of algae start to take hold.
So while it doesn't "cause" algae it absoutely provides an exellent place for algeal to thrive.
It is highly unlikely that a 6 inch piece is holding up a skimmer is going to become an algae farm because there is unlikely lighting pointed at the skimmer.