"It is coming."
Basically, there is no present continuous in french, we will either say the verb in the present tense, or if we really want to point out it is happening right now, we will add "être en train de ..." with the infinitive tense verb.
eg: he is eating: il mange or il est en train de manger they are sleeping: ils dorment or ils sont en train de dormir
so it is coming: ça vient or c'est en train de venir
"Arriver" can also mean "to happen."
He (il) can't happen, but that/it (ça) can. "Ça arrive," means, "That/it happens."
E.g., "Il arrive quand il ouvre la porte." (He/it arrives when he opens the door.) vs "Ça arrive quand il ouvre la porte." (It happens when he opens the door.)
I hope this helps! :0)