"He said" is a simple statement. What kind of statement is: He said Mark didn't like Susan. "He said" can be called a sentence. So can "Mark didn't like Susan." Yet this is neither a compound or complex sentence. What is it called when two different subject/verbs are co-joined without a semi-colon or the usual and, or , but, yet, etc.
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Think of the sentence like this:
He said that Mark didn't like Susan.
So it's actually a complex sentence.
The two sentences are identical except for their complementizers. When you omit the "that", you're using the empty complementizer.
Note that you need to use an agentive ambitransitive verb in your main clause if you want it to be grammatical without the other clause. Your main clause ("he said") by itself just seems awkward if not ungrammatical (although say can be used intransitively).
A better verb would be noticed:
Mark didn't like Susan.
He noticed [that] Mark didn't like Susan.