I doubt they had the Sea Grass installed probably inherited, doubt very much that anyone who went to the expense of having Sea grass would treat it is such a manner the vast majority of people who install it tend to take care of it as they know its cleaning limitations.
In this instance these people are obviously filth merchants and probably mentally unstable in one form or another, anyone who leaves mess like this lying around cannot be right in the head......clearly you need to refuse it, it's up to you if decide to give the reasons why,