Fiery mucus is one of many types of slugs that you can (and should) receive in Slime Rancher 2. They can give you fire which you can either sell for gold or add to your collection.
It is not so difficult to find fiery lines, but you will have to wait a bit before you can face them. This is one of the slugs in the game, inaccessible at the beginning of the game.
The next leadership will tell you where to find fiery lines in Slime Rancher 2.
where to find and get fiery lines in Slime Rancher 2
Their name alone should tell you that fiery lines are often found only in areas with steam or high temperatures, for example, inside the volcano.
You can go to Amber Valley in search of fiery slates. You can find the location by interacting with pink proud which you will meet in the northwestern part of the rainbow fields.
As soon as you meet pink proudly, you will need to feed it food. It can be any food, all you need is to feed it, and then it will be divided into several pink slips.
There will be a geyser below, make sure you are standing on a geyser, and he will lead you straight to the rock. From the cliff towards the left side you will see the device, interact with the device, and you will be teleported to Ember Valley.
As soon as you are teleported, go to the far west from the place where you were teleported. As soon as you go to the far west side, you will face a hot region. There you will notice several types of lines, but not fiery slates, since they are in the far part of Ember Valley.
To get to the back, it will be much easier to use a jet satchel. You can unlock the area by finding several shining ores in a coal valley. As soon as you get to the further part of the smoldering valley, look for large pools filled with fire/lava.
Inside the fiery pool you will find fiery lines. From there you can just take them and get as many fiery lines as you want.