Do your self and your elbows a favor if it is, in fact, dried algae(sometimes called pond scum). Use a diluted, acid based cleaner. Toilet bowl cleaner, the cheap kind you can find at any janitorial supply house... Smart & Final comes to mind.... Mix it up in a 5 gallon bucket and get a long handled pot scrubbing brush to slosh it on. It will literally disappear before your eyes. Hose it down with fresh water, you're good to go. Acid will remove anything biological in nature.. If it has a petroleum-organic character, such as the skid marks from rubber bumpers... then Acetone is the right product.
That sermon completed.... that looks like old paint has simply worn off...not new biological growth. You'll know immediately, because toilet bowl cleaner will not work.. nor anything else... to remove bottom paint. Stripper or sanding are your options... but heck.. you should be able to just follow the instructions on your new bottom paint label for apply it over existing painted surface.