hi dave its not yeast. its bacteria culture from the yogurt. possbily lactobacillus. they will ferment the sugar into i think lactic acid.
from google:
Yogurt typically contains two main types of bacteria: Streptococcus thermophilus and Lactobacillus bulgaricus. These bacteria are added to milk and ferment it, producing lactic acid which thickens the milk and gives yogurt its characteristic tangy flavor. While these are the primary cultures, some yogurts may also contain other probiotic bacteria like Bifidobacterium
as for the video, that does not look appetizing at all . it would prolly be better with scobi.