my cockatiel has that at the moment. He's on nystatin (sp?) from the vet.
It didn't seem to be quite enough, so I got to researching. Found a few more ideas, tried them, they're working!
He's really began responding well when first thing in the morning (before food or water offered) I hold him upside down and massage the crop till he brings up all the yeasty mucus (and leftover food) he's accumulated overnight. Then give him his dose of nystatin, wait half an hour, then offer him water (first day was water with alka seltzer in it, a home remedy), then wait another half hour. Then I give him his seed with probiotic yogurt and grainivore rearing mix in it, and change the water to one with vitamins in it.
Today is his second day on this routine and making him empty his crop in the morning and slowly starting his day with food has made him go from vomiting mucus/food up to 12 times a day to not vomiting at all! (apart from the forced morning vomit!). His weight seems to be more stable too!
Apparently an empty crop in the morning helps because when they eat new food it becomes contaminated by the old food in the crop, so nothing gets fixed. Treating an empty crop (with nystatin or the 1/2 alka seltzer to 1/2 cup water crop flush) if far more effective. Also waiting a while before feeding after treatment lets it do it's job better.
Here's the website I used to the alka seltzer, apparently it also helps break up lumps like yours has
http://justcockatiels.weebly.com/sour-and-slow-crop-remedies.html
note I used the massage/non invasive way to empty the crop rather than using the tube. And let him drink the alka seltzer mix because he was willing to, rather than force it down his throat.
I'm sure it would work for a chicken too, they're tougher than a cockatiel. I would treat my chickens this way if it happened to them.