When there is an error/issue with blocks at build time, the diagnostic viewer can allow the blocks which cause the error to be highlighted with a red border. Similarly, some functionality allow blocks to be highlighted in yellow, like when highlighting a Scope block or when tracing the source of a sample time.
What I have experienced is that Simulink changes the border to red in these cases, but then never properly removes the red border or highlighting...
So what I believe has happened on your Simulink model, is that there was an error in the past which changed the border of the Powergui block to red, but although you have solved the error the red border was never changed back to black automatically by Simulink.
You can manually change back the border color by right clicking on the block > Format > Foreground color > Black.
