Are you fermenting or just getting moist in frig over night? When you ferment it will have to sit for days being stired and when it starts to bubble and has a yeasty smell its ready. It can have a white looking film on it but don't feed anything that molds. Prevent mold by stirring a few times a day making sure you scrabble the sides good.
I keep the feed in the fridge up to 3 days so obviously the first day's feed isn't as ripe as the 3rd day's. I don't have a good feeling about feedling something really yeasty to them yet. I may change my mind but for now this has been working for me. For live cultures I've been using the Greek yogurt separately for them at dusk. I read giving them something that begins to mold isn't good for them so I'm not completely convinced yet. I won't even feed them produce if it looks like it's beginning to turn or mold.