Liver enzyme (alt only) abnormal at 68. Was normal previous two yrs. Hep. Panel negative, ultrasound normal. Should I worry? Moderate drinker (<14/wk)

Maybe. Ask 4 questions about the elevated value. 1. Is it a real result? 2. Is it significant 3. Is the cause obvious. 4. If not what can i do i would stop drinking for a month and go back and have another test. The best person to talk to is your doctor who knows your full history.
Minor. Because the other liver enzymes are normal, the hepatitis panel is negative and the abdominal ultrasound is normal, this sounds like a minor situation. Nevertheless it may be worthwhile to omit or reduce alcohol consumption for 2 to 4 weeks and the recheck the alt. Otherwise visit a hepatologist for evaluation.