Taking Constantinople is a thing, taking over the whole Balkans are another.
First, note that I would be talking more about 670 siege, as the 718 definitely lacked material, navy and logistic to do so.
If by grace of ASB, Masalmah still manage to conquer the city, I could likely see a reconquest by Romans using the divisions and rebellions in Caliphate. They would probably be unable to take back Anatolia in the same row, but they had the naval domination and could easily blockade Arab defense or ravitail.
Now, a 670 conquest.
At the contrary of Anatolia, Africa or even Spain, Balkanic Peninsula is a really "open" region, that doesn't present a "natural" way of invasion and conquest, something that could make the posterior raids less efficient in that matter.
The Dinaric Alps poses another issue, not unlike the Cantabrian Mountains, and I think it could likely host Asturian-like Roman successor states in Balkans along with other in Africa and Italy in a worst-case scenario.
More likely, you'll end with a more decentralized ERE, centered in Ionian Sea, but who would preserve a naval predominance, that Lombards wouldn't have the possibility to immediatly takeover (but an African split is still possible, critically without an Arab pressure on it).
An Arab Thrakia (aka Saloniki/Sofia/Burgas) would be as noted hugely hellenized and open to revolts and appearance of an Islamo-Hellenic culture. I doubt this could go much far more than Danube (that is admittedly, a perfect border) and Belgrade, more or less like 1500 Ottoman Empire, minus Greece.