Most northern powers have been looking toward the thawing north passages for decades. The shortest distance from Portland, Maine to St Petersburg is about the width of the US. Greenland is of huge strategic value, holding a similar value to Alaska. The shortening of shipping lanes will shape global economics, as well as stoking the fears of other powers establishing footholds in northern regions, for the foreseeable future.
China aggressively projects power, with a willingness to spend a large amount of effort (sand islands) for footholds. Notably, by the time the evidence of these artificial islands was observed, it was too late to reconsider. Russia does the same, when they have the resources. The US is not used to doing this and has historically fared poorly. Ofc there is worry.