As the question describe itself "What is the core difference between algorithm and pseudocode?".
I think all the other answers give useful explanations and definitions, but I'm going to give mine.
An algorithm is the idea of how to obtain some result from some input. It is an abstract concept; an algorithm is not something material by itself, but more something like an imagination or a deduction, a thing that only exists in the mind. In the broad sense, any sequence of steps that give you some thing(s) from other thing(s) could be called an algorithm. For example, if the screen of your computer is dirty, "spraying some glass cleaner on it and wipe it with a cloth" could be said to be an algorithm to solve the problem of how to obtain a clean screen form a dirty screen. It is important to note the difference between the problem itself (getting a clean screen) and the algorithm (wiping it with a cloth and cleaner); generally, several different algorithms are possible to solve the same problem. The idea of complexity is inherent to the algorithms itself, not the problem or the particular implementation or execution of the algorithm.
Pseudocode is a language to express algorithms. Since, as said before, algorithms are only concepts, we need to use something to express them and explain them to other people. Pseudocode is a convenient way for many computer science algorithms, because it is usually unambiguous, easy to read and somewhat similar to many programming languages. However, a specific programming language like C or Java can also be used to express and algorithm (it's just less convenient to those not familiarized with that language). In other cases, pseudocode may not be the best way to express an algorithm; for example, many graph and tree algorithms can be explained more easily with drawings or diagrams. In the previous example, the algorithm to get your screen cleaned is probably better expressed in a natural language like English, because it is simple and specific enough for that case.
Obviously, terms are frequently used loosely and exchanged depending on the context, and there's no need to be nitpicky about it, but I think it is important to have the difference clear. An algorithm doesn't stop being an algorithm just because it is written in Python instead of pseudocode. Pseudocode is just a convenient and widespread communication tool to express them.