I wouldn't ever recommend it. Unless someone here can give you special info about cleaning the copper...its ultra-sensitive for an octopus or any cephalopod. My advice...would be to mature the tank also if you plan to use it.
The tank has always been mine. Once used as a reef habitat, now used as freshwater. The particular medication is called Quick Cure, the "contents" label lists only two ingredients: formaldehyde and malachite green.
From what I'm been able to find out after reading a couple of articles, Malachite Green doesn't contain copper - in fact, it's an aniline dye named after the mineral malachite. It's used as a fungicide for fish and does dye things green.
However, it's not considered safe for invertebrates, although it doesn't contain the copper so fatal to octos.
The toxicity of Malachite Green varies with a number of factors including species of fish and its size, and less with prevailing water conditions. Malachite Green can be used at concentrations of 0.05-0.15 ppm. Used at 0.05 ppm, most species can be treated with little if any toxicity problems. However, care must be exercised when treating known sensitive fishes such as dwarf cichlids, barbs, tetras, gouramis, livebearers, catfish, loaches, mormyrids and scaleless fishes. Keep the fishes being treated under close observation and stop treatment, filter the water with activated carbon and perform a water change if any undue signs of stress are noted. AmQuel can be used to reduce treatment concentrations. Caution must always be exercised when using this product at dosages higher than 0.05 ppm. It is recommended that the literature be consulted in such cases for additional information on treatments.
Water conditions in general do not significantly influence the toxicity of the drug and therefore are not prime considerations for altering treatment procedures. When the drug is used in short term exposures, Malachite Green may tend to be more toxic to some species in warm water than in cold water. Preliminary experiments have demonstrated that hardness or pH of the water has a negligible effect on increasing or decreasing the toxicity of the drug.
The effect on marine invertebrates is presently unclear and is not recommended. Until such information is available, use extreme caution if using Malachite Green in aquariums with invertebrates."
The big thing is: does it stay in the silicone, as copper does?
The ich medication was used once about a year ago. The silicone does have a light shade of green to it, though requiring close examination to reveal such. The amount of medication used was half the recommended volume, as a school of clown loaches was occupying the tank.
It really is hard to say...I've used Malachite in freshwater tanks quite often, even ones with crabs, etc in them, and had no problems...it is just that cephs are so dang picky about metals.
If you have the time and inclination, it might not be a bad idea just to replace the silicone...I know it is not a lot of fun, but...