You should not worry top much about your Paph... It looks pretty fine!
However, I would suggest to repot and add new substrate, as the one it is in looks like starting to decay... This would also let you control better what you are doing! Especially, for slippers it is a good practice to always repot them to new substrate after purchase!
Now, a clarification regarding adding dolomite or shells... This is not to add calcium or magnesium as fertilizer! Ok, yes, of course, once here the plants will use I as we'll... However, the main reason adding these is to increase the pH of the substrate around the roots! This is very important for several species of slippers, which would perform badly if the substrate is too acid!
Nevertheless, you should learn firs which species need it, as it could have the opposite effect as well... Paph. venustum is not a calcicolous species, therefore adding oyster shells or dolomite or any other calcareous rock is not necessary, and would actually negative!
Here is a page with good information about slippers: slipperorchids.info