'mu' is a letter in the Greek alphabet. It looks like this: μ If you need labels for quantities or variables in math or science, you can use any letter from any alphabet you like ... just be sure to explain what the symbol stands for somewhere on the page where the equation or formula is.
I've seen 'mu' used to stand for 'micro-' when writing units. For example:
Sample Response: The water is cooler than the sand because it has a higher specific heat value. This means that it takes longer for the water to increase in temperature, making it feel cooler than the sand, which warms up more quickly.
