What does the M in μM-Sux stand for?
You are right assuming that μ is a metric prefix, and that the number denotes the concentration of suxamethonium (suxamethonium chloride, I presume). However, this is supposed to be small caps "M" (as $\rm \small M$), not capital "M" (as $\rm M$). Also note that use of the term "molarity", as well as its notation $\rm \small M$, is discouraged [1, p.27]: 


  The term molarity and the symbol $\rm \small M$ should no longer be used because they, too, are obsolete. One should use instead amount-of-substance concentration of $\ce{B}$ and such units as $\rm mol/dm^3$, $\rm kmol/m^3$, 
  or $\rm mol/L$.


And, of course, $1\,\rm {\small M} = 1\, mol/L$.

Also, check out the answer to a similar question "What does the unit M stand for?"

Reference


Thompson, A.; Taylor, B. N. Guide for the use of the international system of units (SI). NIST Special Publication 811 2008.