To simplify this muck is underwater compost. In most cases rotting out vegetation and algae. Constant use of algaecides will build it up and feed the cycle, throwing everything out of balance, mainly the beneficial bacteria and plankton blooms. Resulting in crystal clear water and algae blooming with aquatic growth enhanced with unimpeded sunlight and heavy fertilization from the muck.
There are several remedies, but no quick fixes. One is to reintroduce beneficial bacteria, also a muck eater. That will take away much of the algae, naturally over time and slowly eat up the muck. You just want to bring the bottom of the ecosystem back in balance.
And yes that black bottom muck in incredible garden feeder. It does the same thing in the pond it does on the land. Feeds vegetative growth. In the pond the dead and decaying algae feeds new growth, a cycle. On land you compost dead plants and feed them back. Good thing about the muck is it is never hot, does not contain weed seeds. However I did sprout some cattails by accident one time. I use it in a blend for my aquatic plants like marginals and lilies. It is naturally what they would feed on.
Aeration helps, and floating plants and or a dye to knock down some sunlight. The number one call in my business is algae. I am just not a big algaecide fan, and this is why. It's a temp fix with long term potential problems.
