What is this blue stuff?

Last night I cleaned all my brushes using the Sephora deep cleaning cleanser like I always do. I left my brushes overnight on a paper towel to dry and when I checked them just now, I saw this blue stuff underneath one of my brushes. I've had this Mac brush since 2012 and I've never seen this before. This is also the only brush this blue stuff appeared under. What could it be?