The blue looking stuff on your cars battery terminal is just corrosion caused by copper sulfate but when it’s more white and you have aluminum clamps, it's caused by aluminum sulfate.
The blue and white corrosion are caused by a reaction between the dissimilar metals of the clamp and terminal, aided by electricity and some sulfuric acid from the battery.
The Blue and White Corrosion on the battery terminals is also caused by hydrogen gas that's being released from the acid in the battery and when It mixes with other things in the atmosphere under the hood it produces the corrosion you see on the vehicles battery terminals.
If the corrosion is just occurring on the negative battery terminal then it can sometimes mean that your battery is is being undercharged.
And if the battery corrosion is on the positive it usually means that your system is overcharging the battery and if the corrosion is on both sides then it could mean your battery is going bad.
You can use some baking soda and water to clean off and dissolve the corrosion but be careful not to actually touch the corrosion with your hands or clothing because the acid can eat through clothing and hands.