How do I get rid of this in a small pond short of draining it and starting over?

Herbicides with active ingredients bispyribac sodium, fluridone, penoxsulam, or flumioxazin (in rough order of cheapest to most expensive options) will kill it. May need to reapply to keep it under control through the end of the year.

20 lbs of tilapia per acre can work if there aren't lots of largemouth.

65 lbs of goldfish per acre can work if there aren't lots of largemouth.

The root of the problem is excessive phosphorous so it'll keep coming back until that's addressed. You can use phosphorous binders, phoslock is an expensive but well known example, aeration especially down at the pond bottom, and apparently some of the muck-clearing bacteria treatments do actually help to my surprise (but they work best when combined with aeration - they're mostly aerobic bacteria). Alum can bind nutrients but it's not as widely accepted for that purpose, it's more of a clay/mud clearing tool. And, you can drain the pond, dredge the sediment/sludge out and start fresh and new. Duckweed is one of the tough ones to deal with.

Thank you for the advice!

Great advice, I have a question 20 lbs of tilapia per acre is that 20 1lb fish or fingerlings. Same question with goldfish .

From my understanding when getting this kind of advice on the Pond Boss forum 1 lb of fish represents a single 1 lb or 1 lb of smaller fish . Though sometimes the advice for fingerlings will change depending on the what is already established in the body of water that will possibly feed on what ever fits in their mouth . When getting advice on grass carp the advice for minnows gets more lbs per acre because of attrition .

Derrell's got it. You get more bang for your buck with fingerlings but they are at higher risk of predation. Your nearest hatchery or fish truck delivery route may not have size options on the species you're trying to stock so if all they have is small fingerlings and you have lots of hungry bass in the pond you should probably stock a bit heavier to compensate for predator loss.

Also, remember on tilapia that they will probably die in the winter (less than about 55 F degrees is lethal to them) so you'll have a couple weeks of fish kill to look at in winter and you'll need to restock them in the spring if you want their service to continue next year.

Sit over it with a 12 ga and a half dozen gaddy dekes in January.

It all depends on how much DW you have. If not much, you can spot treat it. If its an infestation, nuke about 1/4 of the pond at a time. If you do the whole thing at once, the rotting vegetation robs O2 for the water generally resulting in a fish kill. The fish need somewhere to go where there is still O2.