Red eard slidder shell spots are different color like grayish is this bad?

my turtles are healthy they eat all their pellets i bought them with their shells like that. i liked it i thought it was like a mole something to distinguish them by. but recently ive been told that its bad that they have just that part of the shell like that i was even accused of not cleaning their tank out properly i ure i do clean their tank once a week. and i make sure i used distilled water. i bought them on the curb on my main street. the man had them their for about a week before i got my will up and bought my little pets. i hope this isn't anything bad i would appreciate a quick response. and also a way to help them. and proper care of their shell. were i live we have a vet but theirs only one and he only works in emergency care with the animal shelter. if ppl do get appointments with him he charges exceedingly huge amounts of cash. so i would appreciate any help on taking care of their shell.