Why is my string potentially unsecure in my iOS application?

I am initializing a mutable string and then logging it as follows.

NSMutableString* mStr = [[NSMutableString alloc] init];
mStr = (NSMutableString*) someTextFieldIbOutlet.text;
NSLog((NSString *) mStr);

The code works and runs, but I am getting a strange warning (in yellow):

Format string is not a string literal (potentially insecure).

Why?