The maintenance of an Overweight rating by Morgan Stanley suggests a continued positive outlook on Mirum Pharmaceuticals, indicating confidence in the company's fundamentals or growth prospects. However, the reduction in the price target could reflect a reassessment of the growth expectations, possibly due to new data or market conditions. This adjustment is likely to have a neutral to slightly negative short-term impact on the stock price as it reflects a mix of continued confidence and a cautious stance on the growth outlook.