What exactly does Eclipse do here for the symbol CreateDefaultSource?

If I click on a symbol and click on it for a short time, it obscures the background with a fuzzy light gray. Disabling a symbol does not immediately lead to its disappearance.
I use the Color Theme plugin to provide syntax coloring. I thought it might have been the background color of "Content Assist", but I changed it to general prefixes, and I still get it.
And perhaps a slightly more egregious example:

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