This is a common question we get as houses with a concrete slab abutting their brick wall all need an expansion joint. This joint is there to stop your house and concrete slab cracking. If you were to follow that joint down, you'd hit the soil. The bricks would also most likely continue down to a solid concrete slab. I can't see any reason why mice or even ants could access the inside of your home via that route. I would, however, be more concerned about that weeper hole in the brickwork. Does it have any mesh in it? If not, that could potentially be an access point for both mice and ants. If there is nothing in these holes, I'd suggest inserting Weepa Termite Protector Weep Hole Screens. They'll help exclude mice and are also impregnated with Bifenthrin which will deter ants.