Looks like some reactions to metallic content in the stone Ive seen in metallurgy texts. The other possibilities are lichen, bacterial, fungi... regardless which I:d remove it until I knew for sure because it could have a very devastating effect on your entire system, that is any live inhabitants and anything else inside your tank. This can get very costly when pumps, heaters, or filters have to be factored in. Some metallic reactions are rather odd and some are toxic as they oxidize, or whatever its called when metal begins to corrode as it does when rust forms.
Every possibility I can think of is as likely to have a negative impact rather than a positive one. If you learn it isnt harmful later you can always place it back into the tank once you know definitively.