I have a small pontoon boat that I keep tied up at my pier. It has four 12" pontoons that are 12 feet long. There are two on each side of the boat and they are fairly close together. Maybe only an inch or two of space separates them. I leave this boat tied up the pier most all year round. So you can imagine what kind of gunk is on the pontoons. The algae gunk changes color from a dark blackish green to a pale color after the boat is out of the water and dries. But it's still hard to get off.

I've got the boat out of the water now, and sitting up on pipe jacks ready to clean. I've purchased some aluminum cleaner (Aluma Bright by Bosh) which is basically an acid. I've never tried this method before, but hopefully it will be better than what I did last year. The recommendation on the bottle is to dilute 50% with water and spray on till wet. Then let set no more than 10 minutes and pressure wash off. I haven't tried it yet but it's supposed to foam up as in cleans the aluminum.

In addition to the algae there are some small hard white flecks on the pontoons. These must be scrubbed off with something. Last year I used good old fashioned elbow grease and some scotch bright but it turned into one pain in the ass job. Also, it's darn near impossible to thoroughly clean between the pontoons because they are so close together.

Does anyone have any suggestions as to a better (easier) way to clean the pontoons other than what I have described?

By the way, my boat is an Aluma Sport 612 made by Hotwoods. And I just love the boat.

Not sure if this would help you. Sounds like this cleaner (or a similar called alumabuff) and a power washer is a good start.
If you have to grind the barnacles off you might consider a media blaster. I don't know what the best media might be as there might be something a little softer than sand to use a abrasive to break loose the flecks.

If the flecks/hard depositis are iron or calcium/hardness spots then one of the stronger lime removers (CLR Calcium lime rust remover) or WINK brand iron removal which is just hydrofluoric acid might help loosen the debris first.

I can't get a youtube video to embed anymore. Did someone get this feature working again and what am I doing wrongly?

Here is the link:

Pontoon log cleaning

another product:
Purple Power

I'd be tempted to try a big DA or angle grinder with a ScotchBrite pad.

So far, I haven't found any way to put a scotch brite pad on my angle grinder.

This should give you a few ideas...

https://www.3m.com/3M/en_US/metalworking-us/resources/3M-abrasives-for-right-angle-systems/

Not sure if this will help ya or not. We use dry ice for cleaning machines at my work. We rent the machine and the dry comes in huge tubs ( 1.5 tons each ). Makes the after clean up nice also cause it just melts away.

They also have soda blasters, that uses baking soda as blasting media.

The Aluma bright should give you they results you are looking for. First wash it, to get dirt layer off.
One Tip - do not let it dry on the aluminum, do it in a cool shaded area. First thing in morning, before sun-up is good.
Keep it wet for up to 10 minutes and rinse.
The best way to apply, is with a cheap hand pump sprayer, keep walking around to make sure everywhere is treated.

Thanks for all the suggestions. I'll take some before and after pictures. Maybe I can figure out how to post them!

McGuires, smooth cut polishing liquid compound used after their heavy cut liquid...cleaned using a DA sander....Only use the polishing if you want a polished, bright look...finish up with "Sharkskin" pontoon protector...You can find countless youtube videos showing methods for polishing and cleaning the logs. Be careful with the AlumaBrite...some types of aluminum will turn black after the acid wash