One top Wall Street firm is getting less bullish on Micron after the stock's stunning performance so far this year.

Morgan Stanley lowered its rating to equal-weight from overweight for the chipmaker, citing the company's fully priced valuation.

"After being bullish on memory for the last two years, we are moving to a neutral stance. DRAM remains strong but looks priced in as MU is very close to our PT," analyst Joseph Moore said in a note to clients Thursday. "We would rather err on the side of caution in an environment where we can see storm clouds on the horizon."