The green "s c u m" you see in an aquarium is called a spirogyra b cladophora c nostoc d volvox

b. cladophora

Explanation:

The cladophora algae is a type of green algae that grows attached to rocks or wood that are underwater. It forms short, rigid green filaments that branch, it's smell is similar to mushrooms. The cladophora is introduced in aquariums via contaminated plants or equipment, therefore to prevent it's formation it is necessary to clean everything before intoruducing it to the aquearium. This algae is so resistant once installed that usually the tank has to be re-started.  

c: warm water flows from the western tropical pacific toward south america. that should be the right answer! hope it .