my turtle has a white lump on top of his neck? what can this be?

by stacey
(ottawa,canada)

Hi there i have recently purchased 2 map turtles from a pet store and noticed today that 1 of them has a rather large bump on his neck it is of whitish color? the other turtle seems to be fine? i am pretty sure i am doing everything right as to there aquarium? and heat and such just not sure what is wrong and if it will affect my other one please help?