Yeah, 'heavy metal' is probably the most unfortunate qualification as it's neither heavy, nor a metal.
As long as it's not in the form of dust, it's not a severe concern in terms of health & safety. Selenium toner contains a fairly small amount of selenium, particularly in its working dilution. Of course, discarding used selenium toner is not good practice, but in all honesty, I doubt it will have very serious effects in a typical city sewer system. This is not in any way an attempt to condone discarding selenium toner down the drain! But it's also not very likely that it will make the fishes in downstream rivers go belly up if a dozen photographers occasionally ditch a few hundred ml's of Se toner down the drain in a 100k inhabitant city, nor is it likely that your domestic septic tank will go awry if some trace amounts of Se toner end up in it. It won't breach the skin barrier so dipping your fingers into a working stock of selenium toner won't poison you, and even if you lick your fingers afterwards (obviously, don't!!!) it's not likely you'll get sick or die from it.
In sane use of selenium toner, probably the most relevant health concern is the ammonia odor which can irritate mucous membranes in your airways, and not so much the selenium itself.
Having said all of the above: use it in a sane manner. Mop up any spills and wash the cloth used well so you don't get dried selenium toner and the risk of dry selenium powder floating around in the air (not likely to happen in any significant amounts in any case, but you know, just apply common sense and clean stuff up). Wash hands after toning prints. Don't throw used toner down the drain, but either use a replenished system, or, if your working solution for some reason goes bad (it can happen), take it to a suitable waste collection/treatment facility. Such measures are mostly aimed at further reducing any environmental and health load that is already minimal to the lowest reasonably achievable level, and not so much measures taken to prevent severe environmental and health effects, which are just not to be anticipated with amateur use (i.e. occasional toning of some prints).