The element in my HLT is rusty after using it for the last 6 months or so. It's corroding where the element is connected to the threaded connector.
I drain all of the water out after I'm done with each batch. I'm not sure why its rusting. Is it just a poor quality element? It's a 5500W 240V element that I run with an solid state relay and a PID temperature controller.
I've had a similar issue, and found a thread on another board on the topic. The solution for my system was to use a magnesium anode to help with this. People reported that the rust would go away when they mounted one in their kettle. It is basically just a metal rod that you attach in your kettle near the heating element. It requires another hole in your kettle to mount, but it is a full-time mount that you don't need to mess with once in place. The anode I got off amazon was for use in portable hot water heaters (like on an rv or house boat). Total fix is less than $15 for the few parts needed.
On my heatsticks I have an RTV silicone caulk sealing that area, which seems to work well. Of course, that's a permanent mounting for the element. I doubt it would be that hard to remove the caulk if needed.