That is seriously nasty. It looks like you have had some serious water contamination, which has probably been further contaminated with some sort of algae.
I think you are going to have to drain and clean your tank if you want to get rid of the problem permanently. Below is a description of what I have done in the past on my first C-Hawk, which is similar to your Parker from a size and construction viewpoint. In your case, bypass the part about the inline filter.
The way I have cleaned the tank is to purchase an aftermarket 12V electric fuel pump from an automotive parts store and rig up a plastic hose on the intake side that will reach to the far corners of the tank. Then I raise the bow of the boat and tilt it slightly to one side so the fuel will accumulate in one corner of the tank and pump the fuel out through a standard, clear, inline fuel filter rigged up to the outflow side of the fuel pump. That filter will show you how much crap you are getting out. You will likely need to clean it out often. Also, pump the first several quarts of fuel into a clear glass container to see if there is much water in the bottom of the tank. You can use the remaining fuel (less the water and gunk) in your car or truck. Then put in a couple of gallons of new fresh fuel, drop the bow and slosh it around in the tank as best you can to pick up remaining debris. Raise and tilt the bow again, and pump that out, using the same procedure. That will get almost all the crap out of the tank and the Racor should take care of the rest.
Your best access to the tank on your boat will be through the fuel pick up tube plate. So, when you remove that, be sure to clean the pick up thoroughly, or replace it.
If you have a full tank, it will take well over 24 hours to pump out the tank using this method, so you may want to invest in another solution, such as a commercial fuel transfer pump. But they tend to be pretty expensive.
By the way, that electric fuel filter can be used for many other tasks around the home, like draining lawn equipment tanks, and transferring other fuels. It's relatively slow, but works consistently and safely.