While Crappie are opportunistic feeders ... I'm not sure Perch eggs are on their menu. I'm pretty sure they'd scarf up a Perch fry or fingerling, though. Sometimes it occurs that one specie of fish goes on the nest, then gets run off by weather/water conditions that are not suitable to them, and another specie likes the conditions & takes their place. Crappie don't like being disturbed, and the males guard the nest as best as their size will allow ... but, I have known Carp & Gizzard Shad spawnings to disrupt their efforts.
If the water has cooled off enough to cause the Crappie to back off, they may still make the attempt whenever the water temps get back to their liking.
... cp