Either a star was about to form or it wasn't
A star forms when a sufficiently large mass of hydrogen and other interstellar media collects in a sufficiently small volume. Once this happens, the inexorable pull of gravity will cause a swirling proto-stellar disk, and once densities and temperatures are high enough, fusion ignition and a true star.
The mass required is something on the order of 1e30 kg. You ball is not going to have much of an effect, unless your aliens are playing with a ball the size of Jupiter.
Your tennis ball is insignificant compared to the power of the force (of gravity of the interstellar cloud)
Lets say your tennis ball is 0.1m across and 1kg (not really a tennis ball, I guess). The standard gravitational parameter $GM$ for this ball is $6.674\times10^{-11} \text{ m}^3\text{s}^{-2}$. If you multiply this by the mass of a nearby object and the distance between them, you get the force exterted on this object. Lets say you have a microscopic spec of space dust, 1e-6 kg at 1 meter away. The force on this speck from the tennis ball is $6.674\times10^{-17} \text{N}$, and the acceleration is $6.674\times10^{-11} \text{ ms}^{-2}$. At this acceleration it will take two days to pull the speck to the tennis ball, if nothing else is affecting it. At 10 meters it takes about 63 days.
But surely, the speck has some sort of velocity of its own. It is moving in whatever direction relative to the tennis ball, and the tennis ball's gravity barely affects it. In fact, the tennis ball is more likely to attract another particle through static electric charge than through gravity. It is the electric charge of small specks of space dust that probably formed the seed for planet and star formation.
Conclusion
You tennis ball will not have a major effect on the cloud. If it was going to form a star, then it still will in a few million years. If it was not going to, then your tennis ball is now part of the mysterious dark matter. Dark matter as space litter? So that's why they have all those signs on the highway!