Hemolymph is the fluid which flows through the hemocoel of invertebrates which have open circulatory systems, like mollusks, arthropods, and tunicates. The fluid is like a combination of the fluids blood and lymph in vertebrates.

In short, hemolymph is bug blood. It is sometimes green instead of red because some invertebrates use copper instead of iron in the molecules that carry oxygen through their circulatory systems.

Some this writeup is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/