Hose clamp

[gregzz4]

It's odd that the band is rusting but not the screw. It's usually the other way around with mixed-metal gear clamps where the screw is steel and the band is stainless
It's possible the clamp you have is knock-off garbage
If you'd like to try a full stainless clamp, look for a Tridon with the part # starting with HAS ( HS has a steel screw, HAS is full stainless )
If you're having trouble finding one, try a boat repair shop. You can get AWAB, Trident and Tridon to name a few
AWAB are about the best
As for plastic, I can't think of any I'd trust for a return pump
As for rust in your tank, others have stated not to worry about it as it's just iron