Your tank is in the pre invasion stage
Pre invasion stage says nothing about how your tank will look in a week, or in a year. it says only that every tank ever posted on the web with a valonia invasion started from an easily fixed easily seen emergence, and they chose not to act, or to act in a way that distributes the regrowth potential around the tank.
The reason for posting this is that you can leave it in and indeed it might go away, or a grazer might eat it. But if you opted not to, and if you opted to remove it in a way that doesn't cast regenerative bits around the tank, you've opted out during the pre invasion stage of a classic valonia takeover. In some tanks that single spot never grows, and adds to the diversity of the tank (causes the offers of leave it in, no harm, my crabs eat it and keep it in check)
in some tanks that spot takes over the whole tank and you lose hundreds of dollars of life and years of effort.
the number one way to deal with valonia is outside the tank. lift that, and any other rocks with it, outside the tank and remove it how you will. rinse that area and the rocks out side the tank = keeping control of castings.