Is i-131 mibg approved by fda for neuroblastoma?

No. It seems to have proven efficacy. Fda approval is difficult to obtain, and is not necessary for access to this treatment. There will likely be more progress toward fda approval when there is more consistency in the timing, dosing and other treatments. But this treatment is readily available in several large pediatric centers, regardless of the lack of fda approval.
Yes. I-131 mibg is a radioactive isotope used to assess patients for metastases and spread of neuroblastoma. It can be helpful in planning treatment and or surgery. It is also used to assess results of chemotherapy over the course of treatment.