bubble tip anemone question

[Edmonton Eskimo]

I have a bubble tip anemone under 150 watts of metal halide and I'm wondering why it is getting brown algae on the tentacles. It looks like they are getting covered in that brown algae that gets on your sandbed. It seems healthy, it opens up nicely and still accepts food regularly and it has a maroon clownfish hosting it. My water is good 0 nitrates,.1 phosphates, ca 400, alk 2.8 meq/l. I have never seen this before any help appreciated.