A slurry is when you harvest mould from an existing cheese. So, he took a piece of Danish Blue, scraped out a bunch of blue mould from it, and probably mashed it about in some warm water or milk. Remove the chunky bits. Pour this "slurry" into your milk. That will get the mould spores into your milk. I do this to get moulds for cam's and brie's and blues because I don't make enough of either to warrant buying and keeping the spores in the freezer.
Yes , Jeff is right on the mark , mashed up some blue cheese in warm milk added it to the milk , I don't even pick out the chunky bits , just try to mix it until as smooth as possible.
I would never use enough to justify buying the stuff , it would be outdated and no good before I would use much.
This cheese got great reviews from neighbors and friends , it is long gone now and another is about six weeks old , this one looks to have much more blue in comparison to the last one , will put up a post as soon as I decide to open and eat it.
Cheers , Jim.