How do I get rid of white lines when objects collide?

Hi, I'm building my first games level. An FPS horror. Anyways I am using cubes for the walls, floors and ceiling, not the best technique but again, it's my first time creating a game and I want to keep it simple. But when two or more cubes collide with each other (or go inside of one another), it creates an annoying white line that kills the mood. Is there anything I can do to fix this? It's a minor problem, yet I am totally stuck on what to do.

Here is a sample I took a screen shot of the ceiling:

As you can see I have two cubes flattened out to create a ceiling, barley touching each other it creates this annoying graphical error!