What can a yellowish anal leakage mean? it mostly happens when lying down or before bed. seems like it's just mucus?

Anal Mucus Discharge: When the gastrointestinal tract becomes infected or undergoes frequent irritation, the mucus-secreting cells may begin to produce excess mucus, which is passed through the rectum. Get an evaluation if this continues. Some viral infections can cause this and would be self limiting and resolve as you get better. If it persists, as stated, get a GI evaluation. Best wishes.