If it looks like I envision, and it's been out of the water and dried, you're gonna need some aggressive tools. I would go to Harbor Freight and buy the 5" scotchbrite wheels for a mini grinder. That will attack the algae and not damage the aluminum. I completely de-rusted a steel boat trailer with a couple of those. You'll get real dirty.
I'm an Eighth Day Adventist. On the Eighth Day, God went fishing!