The purple algea is of course good.
The other algea well...we all get an algea growing on the glass every few days, thats normal, mine looks green. Just clean it off with the magnet cleaner.
Now if you feel this is more of a problem than the normal algea growth then we will need more to go on. like;
Test scores for phosphates and nitrates, lighting and age of bulbs, you know a little more info?
Thomas