They’re pouring latex on him to make a false chest. So that they can place the arc reactor prop in him and make it legitimately look like it’s embedded in is flesh and with tears and scar tissue. The latex is colored so they can see where they are applying and how thick the layers are. It will then be airbrushed to his skintone and details like nipples scar tissue discoloration will be added.
Here’s that picture
Then the reactor prop is added to the dimple. Basically the latex becomes fake skin and they tear part of the center open to embed it.
Now you can see how realistic it looks.
I cannot express enough how much I love little behind the scenes things like this.