They make bigger dewatering bags, that is what is used when demucking a pond. I have no idea how to clean them out, I've seen them left in place for 6 months to a year, then cut apart and heavy equipment used to move the dried muck.
https://spillcontainment.com/products/ultra-dewatering-bags-oil-sediment-model/Just do a google search for dewatering bags. That was the first place that popped up that wasn't an ad.
That was a good thought to use one to remove the algae in the pond. You might have to up the water flow to a 4" pump. You will have a LOT of water to move and circulate. I've dewatered ponds to lower the water volume when killing a pond, or getting it ready to renovate. I think for removing algae, it might be more beneficial to have the pump as far away from the dewatering bag as possible, so you aren't sucking up the cleaned water.
