Hi, and welcome to fishforum! Could you please read the link below and provide us with that information so we can begin to help you? http://www.fishforum.com/viewtopic.php?t=882
In the meantime, I have a few thoughts which may or may not be related to your molly's behavior. Sorry if this may sound harsh, but I do absolutely mean the best and only want to help you. Many fish stores are very profit driven and misinform customers to make an easy buck, so it's very possible they ill prepared you for keeping your fish alive and healthy.
1) 74 degrees is a bit too chilly for a tropical aquarium. It will slow your fish down and compromise their immune system. Your tank (with the fish you mentioned) should be at a steady temperature of 78-80 degrees, so if you don't have a heater, you should pick one up.
2) 95% of the time the fish sold by fish stores as "algae eaters" become problematic in a community tank. I'm guessing you have either a Chinese Algae Eater (in the standard or gold variety) or a common Pleco. Chinese algae eaters are aggressive fish who will soon stop eating algae and start eating your other fish. Plecos, while less territorial, get to be over 2 feet, and grow quickly, so he will soon be far too large for your community tank. Do you know which kind you have?