What's the intention for Ptolemy's invasion of Greece in 308

[LeGrandAriel]

Because Cassander became the sole ruler of Greece/Macedon after removing Heracles?

[Paralus]

The two were likely "at each other" for some time before the invasion. It is most likely that they had fallen out already in winter 315 when Ptolemy, a member of the "Coalition of the Convenient", had sought an accommodation with Antigonus at Ekregma during the third Diadoch War. Although it suited for the pair to continue together against Antigonus (along with Lysimachus), the relationship was strained. Further strain will have followed Ptolemy's adoption of Antigonus' propaganda regarding the "Freedom of the Greeks". Clearly Cassander could not accommodate such given his garrisons throughout Greece.

Antigonus, having stitched together an arrangement with the coalition in 311(Peace of the Dynasts), spent 310-308 in Babylonia fruitlessly trying to kill off the Diadoch he'd created in Seleucus. Whilst he was away, Ptolemy played. By 308 he was quite strong enough positioned to see just how far could marginalise Cassander; "freedom of the poleis" being the basis. He was doing quite well until he abruptly removed himself back to his satrapy. The reason Diodorus gives - failure of the Peloponnesians to rally to his freedom call - will have been only a small part of it. The return of Antigonus to the west - as in 316 - will have had far more to do with it. His tilt at empire fizzled.

Ptolemy was a "separatist" only when it suited. It now suited. Again.