Probably an SO2 build up. Some yeasts do this bad and others do this if there is a lack of nutrients. Nothing is terrible and can all be fixed caught this soon.
My suggestion is to get some yeast energizer (the tan powder not white looking crystals) and mix that in well. For some of my wine musts I use a wine whip chucked into a drill to aerate the must slightly in the early stages of fermentation like this and that helps to dispel the SO2.
A painting says a thousand words. But a painting while on good mead just looks funny!