I was there today too. Maybe we saw each other without even realising. Initially I thought these decorations were a bit of quirky fun but then they carried through to the rest of the aquairum, it was a bit over the top.
@PAT: I was there between 12 and 3pm. It's the first time they're doing this Halloween thingy, probably got a bit overzealous.
I got to Sentosa around 1 o'clock and headed straight to the aquarium so would have most likely been behind you.