That is the tiling of the Texture. You see the repetition of the pattern. This always happens with image textures.
You can use a few different image textures that are similar enough and mix them with, for instance, a Musgrave Texture as a Factor.
Alternatively, use a procedural texture.
Simon Thommes made a beautiful customizable procedural brick texture! Here is a link to a demonstration with a link to Gumroad underneath, where you can get it for free!
https://www.youtube.com/watch?v=d5YByejuCXQ
