The answer to this question hinges on the tense of the verb, ἔλεγεν, which in this case, is slightly tricky as the root word, depending on which grammar is consulted, is either, ἔπω, or, λέγω. [I personally prefer the latter.] In either case, the tense is Imperfect Indicative Active - 3rd Person Singular.
Thus we should translate, "he was already saying".
This is the translation of ESV, BLB, NASB, etc. The sense appears to be that the demoniac's actions had interrupted what Jesus was saying. I note that several people reach the same conclusion:
(8) For he said unto him.—The Greek verb is in the imperfect tense, he
was saying, as though the demoniac had interrupted our Lord even while
the words were in the act of being uttered.
Mark 5:8. ἔλεγεν γὰρ, for He was about to say: not yet said, but
evident from Christ’s manner and look that it was on His tongue; the
conative imperfect (Weiss).
Verses 8, 9. - For he said unto him, Come forth, thou unclean spirit,
out of the man; literally, for he was saying (ἔλεγε). The unclean
spirit endeavored to arrest, before it was spoken, that word of power
which he knew he must obey.
The poor demoniac is thus presented as confused and betweixt the irresistible power and command of Jesus vs the control of demons within. [A very fitting picture of all sinners!]