Well, I'm not positive it is a sand sifter yet. One would need to look at the tentacles and behavior. If it has been "mopping" the surfaces with tentacles it is a sand sifter. If it just has tree like tentacles it is a filter feeder. If it was up on rocks or something, it may be a filter feeder.
Not all will release toxins. It is not a big concern for me. Don't pry it off things to harshly, or put it in with fish that might pick at it (triggers, puffers). Watch things like shrimp (especially coral banded). While possible, I wouldn't consider it a major risk at this time (especially if a sand sifter)