I get letters about pond vacs. There are some that work great, but they are not cheap, nor are they perfect. Most of the letters I receive are asking about a cheap pond vac.
There are vacuum cleaners sold at swimming pool places. They look like large swimming pool blue dinner plates with a large hole in the center and attach to a garden hose. The force from the water going through the garden hose pushes the debris into a net that you have attached to the top of the 'dinner plate'. They will rarely do any good because they are best at removing leaves, etc., not clumped up dead algae. I have tried replacing the commercial net that you get when you by one of these, with an old panty hose leg and it works better, but still not great.