Had the same problem in my tank, what helped me was a good clean-up crew. Placed numerous hermit crabs, Turbo snails, aster snails, and a couple of emerald crabs. They did the job for me. The tank is all clear of that brown cyno crud.
While syphoning through a typical sand/gravel vacuum, the 2” syphon tube doesn’t have the suction you may need. Take the tube off and use the 3/4” hose to suck out all of the cyano. Then add chemiclean.