Sorry I didn’t respond last night. Just a couple random thoughts. I can’t exactly say why your slimer is struggling. Although your tank isn’t new new, it may just be missing a little something What kind of lighting do you have? Properly lighting a single branch staghorn is actually quite difficult. There will be shading unless you’re using a mix of lighting solutions that hit the frag from all angles. I actually recommend either starting with a mini colony of slimer OR gluing a single branch horizontal on the rock. While it looks less impressive, you’ll give the coral more surface access to light and it should start to encrust pretty quickly and shoot up some branches shortly after.
Just to touch on your chemistry. While 520 ca is not a problem, you’ll probably see your calcium continue to rise if kalk is your sole source of dosing. While calcium and alkalinity is dosed as “equal” amounts with kalk, most Kalk powders seem to be calcium heavy. Also, organisms which use calcification in our tanks also use 2 other elements. Magnesium and strontium. It’s completely possible that your coraline, snail shells, etc, is using more mag and strontium than calcium to lay down its base which will cause your ca to rise as you’re continuing to dose. I would recommend possibly suspending kalk and use baking soda as your buffer until calcium and mag get back in sync. I personally run Alk 6.8-7.2, ca 410 - 430, and mag 1440+. Remember that alkalinity is the buffer of ph to resist downward change. So while you may see a certain level of consumption each day, that’s doesn’t necessarily correlate into coral growth.
Finally there is also the possibility of a bacterial imbalance. Slimer is a great canary coral that will help you identify potential issues of stability. I’d maybe up your WC percentage a little bit with a good quality salt (I know, lots of opinions on that) and then just keep trying. Slimer is pretty cheap and imo an absolute staple for any reef.
