A skin will form on top of any high-protein cooling liquid - white sauce, casserole, gravies, sauces etc. I use a greasproof cartouche laid on top of the hot sauce while it cools as above.
If I leave it in my blast chiller too long while I'm busy, the tin or bowl sometimes gets far too cold and the paper sticks to the side, and if you try to remove it, it sticks to the side and tears. If this happens you just put your tray (as long as it's metal of course) on the gas quickly just to heat up the sides to release the paper, or if you're feeling flash - get the blow torch out to do the same thing. Yes - that's another use for the blow torch at the back of your kitchen cupboard......
If there's a lot of sauce left on the sheet of greaseproof once the sauce is cold, run your pastry scraper along and put it back in with the rest.
Greaseproof or baking parchment?
As pointed out here greasproof and baking parchment are normally interchangable. Baking parchment is slightly thicker - and food doesn't stick to it, hence it's use in baking. I use baking mats (they're life saving) for this purpose, so have greasproof for other purposes - like wrapping cheese & dropping on top of chilling sauces.....
Can I use cling film?
Check on your box of cling film to see if you can use it against directly against hot food. As Trig notes in the comments below (thanks for that), some cling films leech toxic compounds into hot food, especially those with high fat contents - this is another reason for using greaseproof.
Greaseproof paper placed on sauce to prevent skin forming is called?
Making a round cartouche
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