Could it be, as your article states, a leftover imaging artifact from light correction?

As you might suspect, there is a non-UFO explanation of the apparent flare-up near Mercury. According to Russ Howard, head scientist of the NRL group, and Nathan Rich, lead ground systems engineer, it is simply an artifact left over from the way raw HI-1 telescope data gets processed. Rather than a UFO mothership parked near Mercury, the bright spot is "where the planet was on the previous day," Rich told Life's Little Mysteries. [A History of Recent UFO 'Sightings']

To make the relatively faint glow of a coronal mass ejection stand out against the bright glare of space — caused by interplanetary dust and the stellar/galactic background — the NRL scientists must remove as much background light as possible. They explained that they determine what light is background light, and thus can be subtracted out, by calculating the average amount of light that entered each camera pixel on the day of the CME event and on the previous day. Light appearing in the pixels on both days is considered to be background light and is removed from the footage of the CME. The remaining light is then enhanced.

This works great for objects far off in the distance, such as stars, which don't move much relative to the sun. But it gets a little trickier when trying to account for nearer objects, particularly moving ones, like planets.

"When [this averaging process] is done between the previous day and the current day and there is a feature like a planet, this introduces dark (negative) artifacts in the background where the planet was on the previous day, which then show up as bright areas in the enhanced image," Rich wrote in an email.

Well it was roughly the size of Mercury, or at least the artifact could be superimposed over Mercury without moving outside the borders - so I'm not sure what you mean?

Again, your article provides all the answers. I'm more inclined to believe this explanation than to believe the idea that it was a "cloaked spaceship."

"The pixels which form the two parallel lines are where the circle from the planet and the bleeding pixels (cross-like features) overlap as it progresses across the field," Rich wrote. In other words, because Mercury moves over the course of each day, and because saturated pixels bleed light into adjacent pixels, an averaged image of Mercury from the previous day looks like two streaks, rather than an orb.