Not seen that on car discs ever. Seen it plenty on train discs though - very common to get surface cracking like that on them

there are specs for monitoring them and acceptable/not acceptable limits before you have to change them (pita on a train)
Those discs have got very hot in a particular region, where the cracking is and the darker coloured band.
That is only surface cracking atm. It may very well not get any worse and you can monitor it to see, but I expect you won't want to risk it.
Are they vented discs, or solid/grooved?