The Wet Brick is a block included in the Millenaire mod. It is vital in creating mud bricks. They are created by right clicking the ground with a brick mould with at least one dirt and one sand in your inventory. The brick mould can be obtained by paying for one from the matriarch's wife.
Wet bricks that are placed will turn into mud bricks after a varying amount of time. However, they become mud bricks faster if placed outside under the sun. Hindu villages can sometimes be seen placing wet bricks to harvest the mud bricks for construction. Wet bricks can also be farmed from villages and sold to merchants.
You can take it from the bricklayer's area by taking it off of the floor area.