I've had this in Primary before: it usually shows up around the 3 week mark. There seems to be mixed opinions on here as to what exactly causes it. Many people claim it's harmless and say they've never had any problems; others say it's an infection and should be bottled and drank quickly. My opinion is that it's not normal: something is amidst and it shouldn't be there.
In either case, I would rack from underneath it and leave it all behind. Personally, I've encountered this a couple times and had it go both ways. I've had an infection after bottling with overcarbonated and sour beers. But another time, the beers turned out perfectly fine and actually stood up to the test of time (several months down the road, no signs of infection or off flavors at all.)
You'll get a mix of responses but I think everyone agrees you should rack from below and leave it behind.