Chlorella is a seaweed and contains iodine. Based on what I've read from reputable sites and studies it's not advisable for people with hashi's to take high doses of iodine...it's like adding fuel to the fire from what I understand.
Of course anything in moderation is usually safe
. Eating the seaweed or sprinkinling the powder on your food is different than taking a concentrated tablet etc...
I personally stay away from kelp...it makes my joints feel icy hot and my eyes dry and burning (oddly enough progesterone cream makes me feel the same way). I haven't tried chlorella to see if it would cause a similar reaction or not.