Hi,
At the age of 75, with weight loss and mucus in stools, your mum needs to see a Doctor. The symptoms can be due to mal-absorption syndrome, irritable bowel syndrome, inflammatory bowel disease, diverticulitis or intestinal infections. Stool examination & endoscopy may be done. Consult your Doctor. Take care & good luck.
The answer is based on information provided. Exact advice is not possible without a proper examination and investigations. You are requested to consult your Doctor. Take care and keep us posted.