x2There are a lot of details left out of your questions but.....
Duckweed a macro plant often times confused with algae is an excellent source of protein but mostly found in water that doesn't flow. Creek implies movement of water so the algae could be macro or micro algae and not all algae is created equal but chickens need greens as well as protein so ..... The more apt question is can you get your chickens to eat the algae from the creek? If you can I'd say anything in moderation is usually ok.
One consideration is the creek water and possible contamination from any source that feeds the creek. Normal rainwater runoff is probably ok but if there is alot of asphalt surfaces or industry upstream then I'd say no. Ok course remember chicken feed has lots of stuff in it too.
Average creek, average algae - I'd go with yes in the same amount you would use with any treat.
If you want to use it as more than a treat (once a day in small amounts) then I'd try to determine the type of algae and its nutritional content.