No worries. I've never heard of aluminium being dangerous (or if I have, I don't remember doing so) but I guess it is potentially something some people will be worried about if they have heard it.
Personally, I just find it hard to do so, as, in the grand scheme of things, there are probably many other aspects of my life which are probably way more dangerous than an aluminium buttplug. It's all relative. Maybe it is dangerous, but I've heard similar things about the silicone/plastic used in sex toys, and obviously there's a risk of STDs with sex, so basically, shoving anything up there is potentially dangerous one way or another. We'd never have sex if we let it all get to us!