How about using a small powerhead (MJ1200 or similar), and you could attach a short length of clear tubing to the intake of the pump and a long length of tubing to the output.... then you could manually use it to target/spot-siphon areas with the most accumulation, maybe 5 gallon bucket full a couple times a week to be replaced with fresh saltwater, until you have made progress. Also I recommend you replace that HOB filter with a quality HOB protein skimmer.