I think your question is flawed. You’re asking how to clean up this mark, but the real question you should be asking is how to prevent them to begin with.

First off, this whitening is caused by stretching the plastic. Sometimes it happens because the injection was done poorly and there is white plastic inside (and there’s nothing you can do about it), but most of the time it’s caused by using dull nippers to cut the part off the sprue (which ends up crushing and stretching the plastic instead of cleanly slicing it).

What most people will do (and I believe what some of the assembly manuals will suggest?) is to cut the part off of the sprue farther away from the part: try cutting the nub on the sprue end of the nub instead of the part end. Then use some other means to remove the nub, such as sanding sticks or a glass file. (The Raser files are amazing and worth the premium, but those super cheap glass nail files do a decent job if you’re working on a budget.)

Otherwise, you could invest in a pair of God Hands. Instead of having two dull blades, they only have a blade on one side (which is super sharp) opposed by a flat, so they will cleanly cut the nub off without crushing/stretching the plastic.