I think you need to find a few more tetra if you plan to keep that remaining cardinal alive and happy... they're a schooling fish, and being solo must be really freaking the poor thing out.
I think your tank can only hold about 1 more inch worth of fish though, so your algae eater may have a significant size limitation. Perhaps a few shrimp would be a good fit, but only if you research their compatibility with the ADF...
Yes, the algae is on the side of the tank, as well as growing on the high heel shoe I have in there.
I got my ADF from a local place here called Fish Haven. And he's super calm, usually hides all the time because some mollies I had tormented the poor little frog :( so its rare to see him away from the cave he and Diego adore.
I had three cardinal tetras, and the other two died shortly after my bettas did. The remaining tetra hangs out with my mickey mouse platy.
algae eaters can get pretty big so you might end up with a giant fish that'll need a whole nother tank in a few months. I have some big algae problems in one of my tanks (i think the light that came with the tank is too strong) and i just rinse the plants and wash it out fairly often.