^ While Excel has been known to work with killing off algae, its not always a sure fire method...
Best bet is to obviously work out where the problem is... is it too much light, too much ferts, maybe high phosphates etc...
Also, what stock do you have in your tank at the moment? since some shrimp can be very sensitive to Excel... infact, i've lost a few which i'm sure was from dosing a bit too much excel when trying to rid my tank of algae before as well...
Another question is, what is your tank setup like? what type of filtration you running? whats in the tank? do you run Pressurized Co2? How long are the lights on etc? the more info you give us, the better the chance we can try and help you solve your problem...