It’s the notion that racism encompasses a huge range of words and phrases which are seemingly innocent but which have obscure racist connotations
Context is everything when it comes to common tropes.
For example, the 2023 incident of Greta Thunberg with the blue octopus in the ‘Stand with Gaza’ photo. A blue octopus is an antisemitic trope, used in Nazi propaganda imagery, as well as elsewhere.
When people complained, Greta Thunberg said she was ‘completely unaware’ of this, apologised and cropped the octopus out of the photo. She explained the blue octopus is a toy used by people with autism to communicate feelings. She emphasised she condemned antisemitism in all forms and shapes. And this was non-negotiable.
This was the correct response. She didn’t double down and say no offence was intended. Or blue octopuses are everywhere, seen everyday, are innocent and have only obscure racist connotations. She accepted offence would have been taken, owned her ignorance and apologised.
Context is everything. The photo was showing solidarity for Gaza at a time of heightened antisemitism. Greta Thunberg is very high profile and influential. She likely has access to PR advisers. The obvious placement of the blue octopus in a photo she’d chosen to publish could clearly be perceived as veiled racism or (at least) uncaring ignorance of antisemitism.
In a different context (e.g. a member of the public holding a blue octopus autism toy in everyday life or a blue octopus in a kids cartoon, it wouldn’t be seen as antisemitic. As the context wouldn’t be a a photo taken and published to make a statement about Gaza.
In the OP’s context, if watermelon was singled out for a specific mention to a Black client, it could certainly be offensive. Only last October, at a Trump election rally in New York, Tony Hinchcliffe ‘joked’ about black people carving watermelons for Halloween in a speech littered with racist tropes. In previous US elections, there’ve been mocking images and remarks about Obama and watermelon. In the UK, PM Johnson was criticised for using it as a racist slur. It’s a very current and heated trope. It doesn’t mean you should never refer to ‘watermelon’. It means a professional, anti-racist service must be consciously mindful of how it’s referenced in relation to Black clients.