I had thought of kurzi but ruled that out because Jonathan said it was very small. If i recall correctly kurzi is the size of amphimuta. It is supposed to occur in Singapore though
A kurzi's measurement FW 18-22mm. the lower figure is within the range of muta/moorei.
Tantalizing but FW spot 3 is usually wide (ie elongate horizontally) in A. kurzi.
Quite a number of these small species in Singapore have probably been overlooked.