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There is only one thing that will affectively take it off. Muriatic acid which can be bought at Ace Hardware and probably many other big box stores is what works. It is an acid that eats the growth away without damaging the gel coat. You must take care because it can fade or even discolor a metal or aluminum trailer and will etch concrete, but it will not harm any plastic, fiberglass or blacktop. Put some in a bug sprayer or spray bottle, wear gloves and work only outside. Cover the trailer with those cheap plastic drop cloths from the hardware store and keep rinsing them as you are letting the acid soak the hull. Soak the hull for 30-40 minutes and you can even use a boat brush or scrub brush to help the process just make sure not to get any on you. It is a form of "watered down" battery acid and will give a slightly burning sensation to the skin. Water rinses it off and stops the burning and water stops the acid from eating the algae as well. That is why you need to start with a dry hull, apply the acid and rinse the hull only when done. I have done this for many years and it took me a few years to get the hang of it because I always rinsed the acid prematurely being afraid of damaging the hull. Do not worry and leave it on for at least a half hour depending on how clean the hull looks. I haven't checked but I am sure there is something on you tube that you could watch.

Jim G.