Stop the Spread of Invasive Mussels Act of 2021 This bill directs the Bureau of Reclamation to provide federal assistance to states with certain reclamation projects to prevent the spread of aquatic invasive species. Such assistance shall include financial assistance to purchase, establish, operate, or maintain watercraft inspection and decontamination stations. In providing financial assistance, Reclamation must prioritize projects such as watercraft inspections and decontamination stations that have the highest likelihood of preventing the spread of aquatic invasive species at reservoirs of reclamation projects; or projects that prevent the spread of an aquatic invasive species to waters under the jurisdiction of Reclamation, including irrigation, reclamation, or other water projects. This bill provides authority for each federal agency on the Aquatic Nuisance Species Task Force, along with state partners, to (1) conduct mandatory inspections and decontamination of recreational vessels; and (2) if necessary, impound, quarantine, or otherwise prevent entry of a recreational vessel. In addition, it expands the membership to include the National Park Service, Bureau of Land Management, and Reclamation.