Looks like brown algae. Otto catfish would sort that out easily. Don't think you get it from too much lighting, more like too little lighting/oxygen in the water. Possibly a lot of nutrients as well but not sure on that one. Quite common to get in new tanks as well. Not saying to change...
I have an 11W clip on light on my 25L tank (came with it) which is good. Not sure if a clip on light would be the best choice for a 120L tank though.
I read somewhere that for low light plants you would need 1.2 watts per gallon, so for your tank that would be just over 38W going by that. I...
I think most of the eggs will be eaten by the Tetras unless they are well hidden. The fry aren't too difficult to keep alive (I haven't lost one as far as I know).
I think a pH of 6.5-6.8 is ideal but not required, that range is best for breeding. Temperature - somewhere around 23-25C.
I didn't mean that every crab needed access to land, just the ones I know of in fishkeeping. You could ask your LFS if they know of any or you could go with the next best thing and get a lobster (a different species than you already have).
I don't know if there are any big crabs around for fishkeeping, the ones i've seen are all around the same size (like red clawed crabs and vampire crabs). They all need access to land though. Maybe someone else will know of one.
You should start seeing berried females around 1 month after adding them. If you don't know how to sex them then the females are larger and have a darker red colour whereas the males are smaller and a lot less colourful/clear looking.