Sea weed lives on water, mushroom lives in land(Different)
seaweed and mushroom are both decomposers(alikeĀ 
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for the record, seaweeds are not decomposers
Seaweeds and mushroom are alike in the sense that they are both incomplete plants. they do not have true roots, stems and leaves.

they are different in a sense that mushrooms as fungis tend to rely on other organisms like the dead log for its food but the seaweeds are algaes, so they make their own food through sunlight.

I hope I helped.
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