RedForeman ©® wandered in from the void and babbled something like:

just curious... why would algae grow?

High light and nutrient levels generally result in rapid growth of
algae. Once it is in your tank, carried in on plants and such, it can
gain a substantial foothold. It is the bane of planted tank keepers.
Kind of like pond snails - only worse...

Careful management of nutrients and possibly bleach dips for the
plants before placing them in the tank are two ways to help eliminate
this problem. ;-)

Try the krib for more info.
http://www.thekrib.com/Plants/Algae/

Good Luck!