Why does an increase in reduced mass lead to a decrease in frequency?
Consider a simple molecule $\ce{A-B}$ composed of atoms $\ce{A}$ and $\ce{B}$ and let the bond connecting them be described as a spring.  Then the stretching and contracting of the $\ce{A-B}$ bond (spring) can be considered as a simple harmonic oscillation and is described by Hooke's law.
${\nu = \frac{1}{2 \pi} \sqrt{\frac{k}{m}}}$
where ${\nu}$ is the frequency of the stretch,  ${k}$ is the force constant describing the specific spring and ${m}$ is the mass of the system.  In our $\ce{A-B}$ example, the mass of the system is described by the reduced mass ${\mu}$, where
${\mu = \frac{m_A m_B}{m_A + m_B}}$
So our equation becomes
${\nu = \frac{1}{2 \pi} \sqrt{\frac{k}{\mu}}}$
From this equation we can see that as the reduced mass increase's the stretching frequency will become smaller.