If it was Weevil, Perospero or anyone else Marco would have easily spotted him on the surface, there's no fog.
The shadow implies that something's falling from the sky on Wano but is taking its time since it's 10,000 meters high, we have seen something similar before when the shadows of Skypiea residents reflected after blocking the sunlight.
So it's either:
1. The giants of Elbaf using a giant beanstalk to land on Wano and kill Big Meme
2. It's shadow of Urouge jumping off the sky island to save the alliance