Insect infestation

I guess you may be talking about the infestation event that happens with the "maintenance troubles" lot trait/challenge.     The insects themselves look like a sort of grey/brown mist kind of thing with little dots moving on the floor.   If you click on them with your Sim, you should have the options to try to stomp them out (which you'll have to repeat about a half dozen times to be successful) or to pay for an exterminator to deal to them.    Hope that helps.