Ashoreline mussel species has a population density of one organism per square meter. will all mussels be found one meter apart.

No.
This method of stating population density is carried out by counting the number of specimen in a given area and then dividing the number of specimen by that area. This does not mean that each specimen is one meter apart. Some regions may have no specimens for multiple meters while other regions may have multiple specimen in the same meter. The population density gives us an average value.

It's either -limiting factors or -scarce resources but i'd go with my first answer since the food and water are abundant they really aren't scarce