Although it is possible, it is highly improbably. But then again, I have seen stranger chain of events in nature than in fiction.
Actually, let me quote Dr. Ron Shimek regarding this topic:
And more:
Brine shrimp naturally occur in salt lakes and not in the ocean where most colonial hydroids and jellyfish reside. So the possibility of jellyfish being attached to the brine shrimp eggs is very minute.
And add to that the sterilization process done by most brine shrimp egg packages, it is really not likely :)
What is probably happening is that they have a hydroid population explotion because they end up feeding the baby brine shrimp to the hydroids. More food = more population.