Sounds like a flea — the fact you could see it so clearly and then it ran back down into your bunny’s fur, that is so…flea! I hate those things. Even though Jack and Vivian are indoor buns, they still can still get them from us and other animals that go outside to inside.
It’s a good idea though to make sure with your vet. I was using Advantage, but Jack’s fleas became immune to Advantage (very strange super fleas). Could be that we treat our cat with Advantage and we’ve created super fleas from that — who knows. So now we use Revolution. Both of those though you CANNOT use the dosage that recommended on the box without a vets recommendation — as the dosage is meant for cats and dogs. . You need to have your vet weigh your bunny (and depending on the age of your bunny too — baby or adult), they will give you the safe dosage to use.
Advantage normally takes care of fleas within 24 hours, and Revolution takes a few days, but it also takes care of mites too. (again, don’t use these without your vets dosage recommendation—plus you need to find out what “bug” it really is, anyway.