Congee and porridge are different? Are congee more fluid?
Porridge is the more general term, and can describe gruel made from oats, barley, rice, or other grains.
Congee is specifically rice porridge, and can vary in texture from starchy and chunky to light and watery. Okayu is a bit on the starchy side.
Japan also has zosui, another kind of rice porridge. The difference is that for zosui, the rice is already cooked when it's added to water/broth to cook. This also makes it more watery.
In Southeast Asia, where English is a prominent second language, many people use 'porridge' to refer to rice porridge by default. Congee is a term used in regions where other grain porridges are common, like UK or India.