The foamy head that forms when primary fermentation is most active. It is imperative that you give your wort enough headspace so that you don’t have a messy overflow, or even worse, a blowout. I once shot the top off of my air lock due to limited headspace. Enough pressure formed inside the carboy and skyrocketed the cap off and yeast/wort got all over the walls of my brew room. The krausen is usually a creamy color and consists of the yeast and protein carbon-dioxide filled bubbles and usually settles out after 3-4 days. It usually leaves remnants behind, among the walls of the fermentor. Krausen is normal, and as gross looking as it is, it represents a healthy and active fermentation.