yes your worms can manage - there are dozens of different little critters that are a natural part of the decomposer community - the appearance of this one [or your noticing them] probably had little to do with the apple core, which wasn't "rotting" but decomposing - "rot" is something that happens anaerobically, material in a worm bin has plenty of air, so it is bacterially decomposed [and the worms eat those bacteria], although if the apple core stays wet enough it can start to ferment, which will attract fruit flies

the best way to reduce the occurance of other organisms in a worm bin is to keep the feedstock buried, and the surface covered with a layer of shredded paper - but if they don't bother you, they're not really a problem

Bill