The street name has nothing to do with the animal. It comes from a type of vessel called a mol.
For centuries, Middelburg had a tidal harbour, meaning the water flowed in and out of the city with the tides. That came with a real problem: every time the tide came in, it brought silt with it, which settled on the bottom. Left alone, the harbour would have been completely clogged within a few years, and no ship would have been able to get through.
That's where the mol came in. It was a flat-bottomed boat that worked back and forth across the harbour, stirring up the riverbed so the silt would wash back out to sea when the tide turned. Someone spent their entire working day doing exactly that.
The harbour here disappeared long ago, and the water has been filled in. Only the street name remains.
We continue to the Spuistraat and follow it southwest. After about a hundred and fifty metres we reach the Damplein, which you'll recognise by the freestanding covered structure standing on its own in the middle.
