The most likely culprit is the butter. If I were you, and making polenta ahead of time, chilling it, and re-heating it as needed, I would make a "base" polenta...that is, polenta, milk/cream, stock, and cook that and chill. When time comes to re-heat it, loosen with stock, and once it melts and becomes liquidy and hot again, whisk in the cold butter to emulsify, and taste and adjust. I would also add any other flavorings (like cheese) at this point as well, and I would keep cheese out of my base.
When you add butter to the base, chill it, and then bring it back to a boil to reheat it, the butter will essentially melt out and separate into an oil slick.
The above method is pretty much how I've done it at every restaurant I've been at that serves polenta. We usually use nicer coarse ground, non-instant polenta, but the method is the same.