My 12 year old daughter was feeling gassy the whole day and now at night she said she passed gas in the toilet but she had kind of yellow and clear mucus coming out. she have no pain , or diarrhea or anything else. what can it be?

Gas is composed of air you swallow feeding and that created by gut germs feeding off the debris in your intestines. Some foods trigger more gas than others and some people have more gas triggering germs than others. Probiotics can sometimes crowd out the gassy germs. The yellow poop means nothing special. This is likely just a transient diet related event.