most sponges will starve to death if exposed to the open air. most likely, your sponge will have at least some or total die off because many lfs guys are ignorant of this fact and pull it out of the water to bag it. This causes air bubbles to become trapped in the tissue, starving the sponge slowly. I wouldnt worry about any toxins leaking into to the tank if it does die, but i would however watch for an ammonia spike if/when it does die. Best to remove it to a
quarantine tank if any signs of die off such as detatching from the rock its on, or just simply "melting" occur. Most sponges are nonphotosynthetic so they prefer the dark areas in the tank with medium to high flow on them. Also, sponges should be fed various filter foods such as cyclop-eze or phytoplankton depending on the species that you have and its particular diet. If you already have corals, most likely the foods you add for them will suffice for the sponges. the rest of the advice i can give you is to keep up your regular maintenance schedule including your water changes sponges are animals too, and need clean, healthy water to survive.
this blocks the water currents that the sponge uses to feed. sponges are great filter feeders! they are easily on par with a protein skimmer! it is tough keeping them out in the open and free of algae.
