[Ziggy,
I have spent the last hour reading about Halimeda on a couple of boards. I came to the conclusion is that it will suck up calcium and it can grow pretty fast. A few people say that they have some large tangs that will graze on it. A lot said they did not think it went asexual but a couple of people said it did in their tanks.
Overall I think if you had good calcium levels it would be a less of a problem. Though if your calcium was higher, then you would probably find the halimeda growing like crazy. Since you said you have a hard time keeping the calcium over 300pm, I would think that the halimeda is not helping. It seems to depend a lot on how much you have of the stuff and how big the tank is. If you could keep it trimmed and raise the calcium levels, then it would be less of a problem. But I guess that would depend how much you like trimming the stuff.

]