Foam is most definitely the beginning stages of algae. There are 3 things needs to grow healthy algae. The first is light and lots of it, so look for holes in your lid or elsewhere. Water that comes into contact with light will begin to grow things. The second thing needed for a healthy crop of algae is nutrients. Tap water in most cities has a ppm of about 200 to 250 which is plenty of yum yum for algae to grow. No need to waste money on fancy strength nutes if your after a healthy crop of algae. Finally, heat is great for a healthy algae crop. Any water that is above 75 degrees will begin to support happy algae so keep your water temp's low.

These things are no joke and will kill ur crop very quickly not to mention foul up the place.