Sir, I recommend you go read/watch tutorials about creating AI and enemies, there are a bunch out there, just give a quick search on YT,
For your question, you normally use an 2DRaycast
, that will allow you to check if the player is visible to the slime, it traces a line between two positions and you can check if it intersects with any object/group/solid/node.
For your slime to follow the player, it depends on what you are working with, that's why I recommend you to checking some tutorials first, and see what works best for you, if I posted some code here, without you having some basics first, I don't think I will be helping you at all.