Relativistic mass is a counter-intuitive invention that modern day physicists left behind decades ago and usually only appears in outdated textbooks and pop science.
Physicists don't use "rest mass" or "relativistic mass" any more, they just use "mass", which does not vary no matter how fast the object is traveling.
The correct equation for a body not at rest is:
E^2 = m^2c^4 + p^2c^2 (Where p is momentum).
BTW: There is only one recorded instance of Einstein ever writing E = mc^2 for an object not at rest. And for an object at rest he wrote E subzero [meaning with a small zero to the right of the E at the bottom] = mc^2, the correct equation for an object at rest. So popular culture has it wrong, ha ha!
