The yellow blob is "Coward". You can get some good items from him. The better ones have lower chance, but he'll be back every time you go there. He's fast, but if you go around that area a few times, you'll see him. He's not very strong so you can get him with a shot or two, even if he's just off-screen.
Those multiplying shrimp guys - Yeah I've seen them get out of hand before. The last form you get is good to clear them away.
Disappear? Like Coward? Or like the octopus which goes invisible?