Perhaps brown mould. Seal the seam.

I didn't think of mold since it's texture looks crusty. I've seen pink slime mold this looks very different.. I'll clean again and re-seal it and hope that helps

Dig out as much as you can, hit it with a black Sharpie, and clear silicone. Clean up regularly.

That looks like the kitchen sink faucet in that photo. What you are seeing is likely the result from wet hands dripping when using the kitchen faucet handle, repeatedly, and that dripped water seeping into the joint. If the water is hard, that brown is from the minerals in the water. Not mold.

The only way to get rid of it is to dig out that joint and put in new. Whether caulk or silicone would be the better choice is hard to determine - caulk is a stronger seal, better for where you need an airtight and watertight joint. But Silicone handles the movement between the horizontal and vertical planes much better.

What some people do to prevent the issue of dripping water at the faucet handle is to install a faucet that allows the handle to face center rather than on one side. Takes care of the problem, and people who do it love it.

Agree that it looks like mineral deposits from water sitting on the surface and evaporating.

Vote #3 for minerals drying on that caulk. I can see the white mineral marks on the faucet, and it looks like there is a white mineral mark on the stone in the right of the 2nd photo (could be a reflection, but certainly the faucet shows minerals spots). Also a few small ones on the backsplash behind the faucet, not just the counter. What you have there is not mold, and it is certainly not "your cleaners are not effective enough" that someone said above.

Looks like minerals from evaporated water. Clean it up with a toothbrush and vinegar.

“Clean it up with a toothbrush and vinegar.”

@Kelly DO NOT TAKE THIS ADVICE. Vinegar is made up of acids that can severely harm your granite. Even if it is diluted, vinegar may still leave acids on your granite countertops. You will have a much bigger problem of permanently etched granite than the problem you have now.