So, It is actually safe to keep coral and inverts in the same QT together if you are QTing for fish parasites as long as there are no fish in the system. Fish parasites need fish as part of their life cycle. an invert can not "infect" an exposed coral skeleton with an existing fish parasite.
It is actually safe to add new inverts and coral to a fishless QT. Because of the life cycle of the parasites. Here is a great illustration originally done by MarineDepot of Ich. As long as you don't take those out of QT before their 76 days is up. You can have a rolling coral/inver QT system safely. (Beware of the water in that system, though)
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Notice that in the infection stage (Theront ) the parasite must find a fish to feed on within 48-72 hours or it starves. That ends the cycle. The reason we QT inverts and corals for fish parasites is that the Tomont / Tomite can release Theronts for up to 72 days. (72+3+1 for safety = 76 days)