in my 29 gallon i noticed this afternoon in my 29 gallon only in the parts of the tank under a 50 watt halide 6700k light. there is gray all around it. like a yellowish gray. it doesn't look like waste, but is there anything bad about it. it appeared since I left for work today. i think it's a form of algae, but I don't know if this is good or not? it is only under a strong light. not on the unlit side of the tank.