I agree with a previous comment that it's good to show that parents don't suspect people who want to harm their children, so many times the parents don't notice and they don't understand the change in their child either. However, Martin is suspicious and I think it's really strange that Stacey won't even listen to him about it. Surely, you wouldn't react like that if your ex told you there was something dodgy about him, especially as Theo isn't involved with Stacey atm. I can't think of any reason Stacey would completely dismiss the idea that Theo could be grooming Lily.
It really annoys me when EastEnders goes with the most unlikely scenario