This is certainly sexual harassment, at the very least. I’d love to help, but I have no real experience dealing with this kind of situation. The only thing I can think of is to log details of each incident, if anything more happens.
The only stuff I could find was on articles about sexual harassment at work, but I’ve copied/pasted a bit that might be useful. If it keeps happening I think you should let the police know, though.

Record;
The date, time and location of the harassment, what happened, what was said and who witnessed the behavior.
Keep copies or take screenshots of any relevant emails, texts, photos or social posts.
Tell a trusted friend or family member what happened and write down the details of those conversations. Not only can they provide support, but they may also be able to provide corroborating statements should you need them.
Keep in mind: Secretly recording harassment can provide valuable evidence, but check your state’s laws before you do so. In some states it is illegal to record a conversation without both parties’ consent. Some company policies also prohibit recording.

I wish you luck, your grandfather is being gross