Place the melon and sachet in a medium sauté pan over medium-low heat and cook, stirring, until the melon begins to break apart and its juice reduces to a syrup, about 5 minutes. Add the heavy cream, bring to a simmer, and reduce by one-third, about 8 minutes. Discard the sachet, transfer to a blender, add the xanthan gum and salt, and puree until smooth. Pass through a fine-meshed sieve into a bowl set over ice. Stir until chilled and thickened. Transfer to a piping bag and reserve, chilled.