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Explanation from the Naval Research Lab:

Head NRL group scientist Russ Howard and lead ground systems engineer Nathan Rich say the mysterious object is in fact Mercury itself. And what we're seeing in the footage is the equivalent of Mercury's wake, "where the planet was on the previous day," as it travels through the solar system on its natural gravitational path:
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.
The analysts say the practice works even better when applied to far-off objects such as stars, which don't move much relative to the sun. But for moving objects, especially planets, the process is a little more complicated. And making matters even trickier is Mercury's staus as the closest planet to the sun.

"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 explained in an email.