Not an issue, especially not before reaching maturity phase/size ...
Rhizophore mangle root system does have sheer puncture power, but the root system typically grows around/over/under/away from barriers, such as the glass pane/seam.
Only real, pertinent issue is once the tree matures heavily, and/or develops root structure(s) which grow into the corner seam, at a perfectly perpendicular angle, forcing it only thru the seam itself. However, the rarity of this, coupled with the proper maturity of the trees root(s) and the underestimated strength of a quality/properly siliconed aquarium, is incredibly rare ... Despite this, one would be well served to protect the bottom corner seam(s) with a blunt piece of acrylic, or similar material, underneath the sandbed; effectively redirecting the root system away from this "weak link". Weak link is purely relative ...