Brown rust like color covering ornaments in fish tank, is this bad? And how do I get rid of it?

For a couple days now I've had brown rust inside of my tank covering parts of the ornaments. It is not a new tank, and his been cycled properly. It doesn't get any sunlight to it but I always keep the light on, I have South American Cichlids in there and they seem to be stressed. It is an 80 gallon tank with a 3 layered canister filter. Thanks let me know.
No, I have no plants, because every time I put a plant in there, fake or not, the Cichlids eat them up. And, I don't know, I kind of use it as a light for the room. I didn't think there was anything wrong with it! Haha
 
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