It's about a mage who was incredibly powerful but died while fighting a dragon, the mage then reincarnates as a boy on Earth. The guy also had white hair before he died as a result of being so strong. that’s about all I can remember.
Here is one candidate and it’s not a bad manwha even if it’s not the one you’re looking for. It’s even completed.
He died in a previous life fighting a dragon. He used to have long white hair and the dragon was happy that someone had reached his level before killing the MC. Then he is reborn in a world (Earth) that is being attacked by monsters coming out of rifts and he uses his knowledge/power to make his mom healthy and protect Earth.