I'm trying to capture wild yeast to make my own sourdough bread. I put prunes, water and some sugar in a sterilized glass jar and aired it daily. After a few days I get a white, fuzzy layer. The smell is sweet and alcoholic. However I don't get any bubbles, only this layer. Do I mix it? Should I wait more?
That appears to be mold and you should get rid of it. I've seen similar methods online, but fruit, water, and sugar appears to be a method of making a starter for home-brewed alcohol rather than sourdough bread.
To make a sourdough starter using fruit, you should use flour and water and remove the fruit. The yeast are only present on the surface of the fruit, and so you only need to "rinse" the yeast off of the fruit. I've seen people shake their chosen fruit with the flour and then remove before adding water. Alternatively, I've seen the fruit soaked or stirred in the water before being removed, at which point flour is added.
Honestly though, you don't need to use fruit. Flour and water will with time and attention create a starter on their own.
Here's an article that shows the progress of an all-purpose flour based starter, a whole-wheat based starter, and a wild grape starter also based on all-purpose flour. They actually found that the whole-grain flour had an easier time attracting wild yeast than the grape starter.