literal translation: there's prawn behind the rock
means there's something "fishy" or hidden intention behind someone's attitude.
speaking about prawn, there's another proverb involving Prawn:
Otak + Udang
"Otak udang" (Prawn's brains) means dumb or stupid.
because prawn's brain are dirty, if you like to cook some prawn, then you know that there are a lot of crap in it's back (not really in its head, but yeah, it's dirty).
it was very difficult to make, I spent two whole nights to made it, so I have to ignore their comments :p
it's not just brains, there's a little twist behind that brains.
so I invented a new proverb is "ada otak udang di balik batu" (there's a prawn's brain behind the rock)
it could means there's a dumb intention behind someone's attitude :p