Consider I have two groups: control and treatment. The mean for glucose variable is written as $\mu_g$
How can I write mean for glucose for the control group and treatment group in mathematical notation?
I want to avoid using superscript e.g. $\mu_g^t$ because that is used typically for exponents
To summarize the discussion in the comments:
I. It is by no means unusual to use superscripts as exponents. Einstein notation is one typical example. To be sure, this can sometimes lead to ambiguities, but in context, with the proper warnings written out, people can sort it out.
II. Sometimes people use parentheses or brackets to distinguish between various sorts of superscripts. Thus $f^2(x)$ might denote the square $f(x)\times f(x)$, while $f^{(2)}(x)$ might denote a second derivative or an iterate ($f\circ f(x))$ and so on. Again, this can sometimes lead to ambiguities, so be sure to spell out what you intend at least once (and then be consistent).
III. You can just use multiple subscripts, as in $\mu_{g,t}, \mu_{g,c}$
IV. If, as appears to be the case here, you only need to distinguish between a small number of cases, a simple approach is to change the variable name. Let $\mu_g$ be the test mean, and $M_g$ the control mean. This is the least ambiguous method, but it is useless if you have many cases to distinguish.