If the red marks look like long racing stripes, it could be bacterial or possibley from a low pH problem. In either case a few large water changes with well aged SW
should help that and clear it up. Check your pH later on in the day and post the results to be sure.
If the redness is mainly around fins and such, it would definately be bacterial but I would start with the same water changes and be sure the fish is fed with vitamin soaked foods to help boost it's natural immune response.
If after a few good water changes there is no improvement over the next week, you may need to QT
and treat with an antibiotic. I would start with the water changes first though.