My son 18 month have slimy poop , semi solid poop from past two month, is there anything to concern?

It really depends on: what your 1.5 year old eats. Many people in different cultures continue to give their children formula milk and little to no solid foods at 18 months. That is not recommended as his colon/ small intestines are developed enough now to demand more protein and vegetables. So the milk has to decrease to 2 cups a day. If he is using solid food consistently then need to see MD for proper workup.