I agree that it is some sort of mold and I would not eat it. While many molds aren't toxic to eat and can make some foods taste great, like blue cheese, the very same mold makes other food, like bread, taste terrible. Even though the fuzz appears only at the top, it is likely that it has penetrated the nice growing medium of pesto. I often make large batches of pesto with summer herbs from my garden and store them long term in the freezer. I usually pour a thin layer of oil on the top to reduce oxidation of the herbs.

Red yeast rice in China, corn smut in Mexico -- molds with a purpose. The USDA chart is a good one for our everyday experiences. Personally, my problems with mold usually stem from the anonymous person who eats from the serving spoon, then dips again.
I have been making a cilantro chutney from Afghanistan that has never molded, from "World Vegetarian Classics" by Celia B. Brown --
Blend/process: 1 bunch cilantro (stems and leaves, coarsely chopped), 1 1/2 Tb light brown sugar, 1/2 cup wine vinegar, 2 hot green chilis, 2 cloves garlic, handful of walnuts.

Agree NO. Mold is very toxic for eating and especially breathing. Fresh products with those ingredients and no commercial preservatives don't keep that long. Next time, make it without cheese, freeze in small batches and add the cheese each time you defrost.

The USDA has a good fact sheet on moldy food:
http://www.fsis.usda.gov/factsheets/molds_on_food/index.asp
It doesn't specifically mention moldy pesto, but it'll give you some information to make your decision.

That would be mold so the answer would be no.

I wouldn't - probably wouldn't kill you or anything but I would be willing to bet the flavor is off if nothing else. And actually, if it has oil that is probably rancid .... yuck

Not to be a scaremonger, but I would not use it.