Did you dry hop? That might be hop particles grouping up on the surface.
A lot of people worry about infections, I read. I have pretty much come to realise that you need to be very, very unlucky to get one. I mean, my hygiene isn't spectacular (not careless, but also not fanatic). I've made somewhere around 60 5-gal batches of this and that and never had an infection yet. Or maybe we just don't have wild bacteria here in Finland - yeah, right ;-)