That looks like crumbled grout. If it was me, I'd have used caulk (the kitchen/bath silicone kind tinted to match the grout shade) along the line where the backsplash meets the countertop edge (but that's me).
If you can find waterproof caulk tinted to match, I'd just get that and run a thin bead of it over the area, then smooth with your (wetted) finger. If you can't find tinted caulk, then mixing grout isn't that difficult at all. Caulk is more flexible and would not crumble like grout, and grout if not sealed will also stain. Caulk generally is waterproof, flexible and won't stain easily. You can check YouTube for basic caulk techniques too.
Check at a hardware store about getting either caulk or grout tinted to match and also see which one is recommended for this type of application.
