All seaweeds can be composted, some need added dry vegetative matter to prevent concentration of the odorific iodides and sulfates. Having a back-yard capacity chipper/grinder can yield the vegetative matter necessary to turn the ocean slime-weeds into something very useful for improving highly calcareous or silicatious topsoils. Shredding, grinding, and chipping garden and landscape debris, then mixing with seaweed detritus, makes fast work of smelly wastes, especially in warm temperatures.
Even in New England, which is the tailpipe of North America, we don't overly concern ourselves with " heavy metal contamination" of either seaweed, or any of our highly regarded sea-run fisheries from the open ocean. There are even now a growing number of commercial composting businesses turning a wide variety of wastes into both organic fertilizers and organic growing media ( aka potting soils).
Routine assay testing is an early warning of " heavy metal contamination", and no one in any part of New England wants anything to do with any matter coming from a well known part of Penobscot Bay that was highly polluted by Mallinkrodt Industries in the 1970's. The pollutant in that area is mercury in the river/sea bed. A large area to seaward from the Penobscot River south of Searsport is off limits to all harvesting of fish, lobster, clams, mussels, sea urchins, scallops, and seaweeds/kelp. The remediation of that travesty is so expensive, even the US Court system has been unable to recover the funds needed.