They were up against each other for a long time with no ill effects. I have yet to see a hammer sweeper he is harmlesss there. I also had a galaxy near them too but after he killed some cool mushrooms and zooz he was banned to the other tank where he is in high flow and his sweepers never come out, he seems to like it growing like a weed.
That gray ric seems to be killing off some shrooms he will be evicted soon.