Can we say frequency is equivalent to mass?

The formula $E=hnu$ is only applicable for photons. In the case of a photon you are right $nu$ and mass proportional.

For particles other than photons we can use de Broglie wavelength which is given by $$lambda=frac{h}{p}$$ For a particle with relativistic mass m and rest mass $m_0$(for photons $m_0=0$)- $$E=mc^2=gamma m_0c^2=sqrt{(m_0c^2)^2+(pc)^2}$$ $$Rightarrow E=sqrt{(m_0c^2)^2+(frac{hc}{lambda})^2}$$ In practice the above equation is not very useful.