To win this challenge, an organism must sustain itself with intelligent food-seeking behavior, and also avoid touching the green cells in the middle--because if anything gets too close to them, they will explode in a shower of photocytes, ruining your species!
A swimming organism is probably the best choice, since there is some viscosity to the substrate. If you have completed the Scarce Conditions challenges, you must know how to make a smart food-seeking swimmer. But here it must be extra-smart, to prioritize avoiding the green bomb over the less important task of eating.
With two stereocytes, one for each behavior, it is possible to adjust their relative strengths, and/or connect the higher-priority stereocyte directly to the flagellos, and get the right balance using only one signal substance (like S1), and without needing a neurocyte. The problem here is the limited range of signal "importance", and making sure green-avoidance is top priority, no matter what food may be near by.
If you give each stereocyte its own signal (like S1 for avoidance, S2 for food), you can combine them with more fine-grained control in a neurocyte, then output S3 or S4 to control the flagellocytes. This is one more cell for your organism to sustain, but the payoff in improved accuracy can be worth it.
Another design that I recently had success with is a swimmer with two segments for seeking/avoidance. Here is how it looks:
Genome: https://www.dropbox.com/s/98hymliier788 ... enome?dl=0
The dark purple segment has two flagellocytes and a food-seeking stereocyte, all using S2 for signaling. These flagellos swim forward and steer toward food.
The light purple segment is for avoidance only; if green cells are sensed, S1 signal tells the side-angled flagellos to swim away from them. If no green cells are near, they rest, conserving energy.
Finally there is the head, or mouth, two phagocytes that can suck up the big juicy nutrients this substrate provides. One phagocyte would work, but there's plenty of room for two, and having two yields better food absorption. Plus they look cool, like bug-eyes!