Okay, if it is your map, then I'd say check and see if that triangular area is a separate block (aka brush) from the surrounding L-shaped piece. If it is, you probably want to use the tool with the multicolored block to play with the texture- try realigning it as Owlruler said. You might also double-check to see if there's any odd lighting situations that might be cropping up, such as something higher up that's casting an L-shaped shadow or something to that effect.
Beyond that, I'd suggest screenshots of the various windows and popups in Hammer so there's more information to work with. Properties, the aforementioned multicolored block tool's popup, and so on.