Answer: I am a microbiologist that usually studies bacteria. However, I too had a growth somewhat like this in my sink. Not as bad though. So being the nerd I am I took a sample an analyzed it in my lab and concluded that it was a fungus in the Fusarium genus. And that makes sense as it is a common infection in sink drains. The bad thing is that in large blooms they can lead to some infections. Though they are rare.