Can I just srub him with an old toothbrush or something? Or should I just leave the algae covered snail be?

you can do eathier. I find the toothbrush method isn't a real big improvement and i just let them be. also if your snail has snail erosian, growing back a shell, or is a baby i wouldn't try it because their shell is vulnerbale (sp)

Yep either way is fine..but I like to see mine without so this is what i do get a old pot put some tank water in dip a very soft toothbrush { a babys one is the best} clean gently taking care not to catch any part of his face etc..keep redipping the brush in the water..then dry him with a piece of kitchen paper then paint him with nail hardner ,I use colourless I prefare it to a colour..let him dry fully before putting him back in the tank...I know a lot of people are scared of doing this but it does no harm to the snails and the stuff you use is not toxic once dried..and it does help if he has started to look pitted...when I get chance I will put a pic up of Sid who is my oldest snail and he had a repair job last night with a piece of old shell that I asked for at the LFS and cut to shape with a modeling drill kit and underwater construction epoxy..he is now as good as new.. unfortunatly I didnt look at the colour when I bought it and now he has a purple rear but he is still scoring with the girls Linda