Can a fetus have extra fluid behind the neck and still be okay and have no disorders or problems?

Yes: while this can be considered an abnormal finding on ultrasound, there are many things to take into consideration. The significance depends on the fetal heart and chromosomes. A simple maternal blood test can check for fetal DNA and be reassuring in many cases. While there are possibly associated abnormalities, you need more information before making any judgements! Best of luck to you.