Need advice on freshwater tropicals

I have a 70 gal. freshwater tropical tank stocked with some 
assorted plants. I believe they are: Ludwigia Repens, 
Bancopa Monnieria (Baby's Tears), Hygrophila Polysperma  
(Water Star) and some other thing. They all did really well 
for the 1st week or two, but have been rapidly taken over by 
a strange algae that grows on the leaves and stems. On the
leaves, it forms into small black spots, and they cannot 
be removed mechanically. Anyone know what is going on? Strong
lighting is not helping the situation.