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.
I do have a protein skimmer. It's still in break in process and I've had to turn it off since I'm treating for ich. I'll probably just have to deal with it until that's over. Don't want to stress fish more.

