how to remove less concentrated pixles and leave the most dense part

i know the title does not make much sense but here is the situation i'm working on a computer vision algorithm and i end up with something like

working with the algorithm is way too expensive and no matter how big the threshold i add there is always noise (that less concentrated lime green)

all i need is to end up with that concentrated pixels and at this point i really need clever idea to save processing

no i don't really need code, just idea would get me going