That's mold growing on the side of your trailer (there is probably more on the north facing side than the other sides. You could use any cloth including microfiber cloth to wipe the mold, but I would use some type of cleaner as well such as a mix of Mr Clean and water and then hose it off. In fact, wash it down with the hose and see how much comes off that way - it might save you some time.

Is it aluminum siding or vinyl siding?

Since it's mold, I'd put on old clothes (because you're dealing with bleach) and protective glasses. Then spray Tilex on it, and spread it around with a big scrub brush -- a long handled soft brush for car washing would be perfect. Let it sit for a few minutes (as long as you can without it drying out) and then hose it off. Quick and easy, and it will kill all the mold spores.

Tilex is GREAT stuff. I use it on my shower stall. I use it on my old scratched up white kitchen sink. I line the bottom of the sink with paper towels, spray them with Tilex, retire for the night, and in the morning toss the towels and the sink is the whitest ever!

But Tilex will bleach out your towels, washcloths, and clothes.

@graywings--We have white vinyl siding.

There are certain products you shouldn't use on aluminum, that's why I asked. You can use just about anything on vinyl siding. Tilex will work well, but it's meant for small jobs like a bathroom, not cleaning an entire trailer, so it could end up being more expensive than something you mix up yourself in a bucket. I guess it depends how big an area you are going to clean.

There are lots of formulas on the internet, everyone has their favorite. I like using a mix of laundry detergent, bleach and TSP in water. Lots of people prefer oxyclean to TSP and bleach because it's better for the environment. Here's a link that has several recipes for cleaners.

The long handled soft brush is a great idea.

Here is a link that might be useful: Cleaning mixtures

I'm sure it's safe to use microfiber cloths on vinyl siding. Microfiber (in an appropriate texture, and clean) is the best tool to clean delicate, easily scratched surfaces like show automobiles, optics in binoculars, etc.

I read in the Costco Connection magazine about a product called Wet & Forget that easily cleans mold, mildew, and algae off practically any surface and is safer for the environment and the surfaces themselves.

I've successfully used OxiClean for soil and algae stains on my painted stucco deck and outdoor cushions, but I had to let it soak on and then scrub quite a bit. Wet & Forget is just like the name; you just hose it on and leave it, so that sounds good to me.

Here is a link that might be useful: Wet & Forget

Now, THAT is an intriguing product . . .