We trailered our sailboat in San Diego so never experienced bottom growth. That is some crazy looking yick!

Don't know if it will work for you but something I have done for many years here. Knotmeter impeller gets a light coating of marine trailer bearing grease at the beginning of every season. Otherwise the pesky zebra mussels will move in and try to build a condo. I use a Q-tip and smear the grease over all exposed surfaces. Might not be 100% environmentally friendly but it is a very small area.

Water temps here can vary as much as 30* during the season. High 40's at the beginning of the season to high 70's even low 80's by the end of summer in the shallow basin. While we get the usual build up of algae at the waterline, the biggest issue are the zebra mussels. They attach to everything without bottom paint. In the fall when hauling the boats at the club, some of the "lightly maintained" boats have to get their bottoms scraped with plywood to get the zebra mussels off so they don't cut the lift straps. We usually have a talk with those owners to encourage them to invest in a coat of bottom paint in the spring.