I did a bit of digging on FW feeder fish for SW predetors. I came across an article that is a good read. It would seem that FW feeders also do not provide the proper nutrition that a SW predetor would need. It might be fine for awhile but will cause problems over the long run.
If it were me I would make a serious effort to train him onto Frozen or Freeze Dried foods. It might take a bit of effort but the results will be happier for you and your fish.
http://www.advancedaquarist.com/issu...003/invert.htm