I noticed that after about 3 years of use, my metal stand is developing some rust spots. Being under a 120 gallon tank, obviously that's of some serious concern!!

Right now they are on the surface of the metal, but I'm worried about it spreading and comprimising the strength of the stand.

How worried should I be? Once the rust is on the surface, is the stand doomed eventually?

How can I prevent further rust? It's funny, even areas away from the sump have some rust, probably just from the humid salty air.

What should I do with the current rust?

When I bought it, they said they powder coated it - do you think it was if it is rusting already

Thanks so much,
Yes, tube steal will get moisture in it an rust from the inside out or you may have scratched it. Saltwater promotes rust on steal surfaces, check out some rust inhibitors and apply.
You should sand the rust down to bare metal and repaint the area. If you cover it with paint or rustoleum or a simular product the stand will continue to rust under the paint.