It can happen. There are so many variables: weather, bather load, FC fluctuations, circulation. A bather can bring algae in on a suit or a float or a toy if they've been in a lake or river. Stuff happens and algae are opportunistic. In my pool, they hide in the (unused) light niche which gets no circulation at all and they will pop out if I'm not paying attention.

Shock the pool to 15 ppm FC, brush the area where you saw the patch. That's been my approach and it has worked for me.