My last job was in the online learning space. In my final interview with the company owner I brought up my worries about this very thing, he looked at me concerned and asked what my side project was about. When I told him it was something to do with NLP he laughed and basically said he couldn't care less as long as it doesn't compete with his product. I wanted to get that in writing but I wasn't in a strong position to negotiate; regardless it was never a problem.