No, the only causes of action they might have against you are infringement or dilution and this is neither. They can write all the nasty letters they want, no court will entertain such an action. It’s simply not illegal. If they tried to get an injunction against you it would simply be a waste of money.
They are worried about it, but they cannot do much about private individuals who decide to use a term in a generic manner. They can only take action against their competitors.
You don’t have to to anything. They simply do not have a case against you. First of all, it’s unlikely they’ll do anything. If they do send you a letter, you can safely ignore it.