It will not spread due to presence of a dead pixel on the phone. If cared for properly then there is no reason for it to spread. If multiple dead pixels appear then it means that there could be undue pressure put on the screen at a particular point by maybe a sharp object or on impact or short circuiting of the board.

Can dead pixels spread?

Dead pixels usually don't spread. They're usually a small fault in a display. If they do spread, you might need to hire a specialist or replace your screen.

Can dead pixels fix themselves?

Dead pixels are much less likely to correct themselves over time, and typically cannot be repaired through any of several popular methods. ... Stuck pixels can often be re-energized by rapidly turning them on and off. If this fails, try applying pressure to the pixel.