I had a similar problem and used bleach/water solution and an old toothbrush to gently scrub it. It worked for me and my meter is still working fine, although can't guarantee the same for your meter.
My meter suggests using a little 'buffer 4' in the probe cap for storage, but again, it may be different for yours.
Loafyboy Reviewed by Loafyboy on . Mildew on my Ph meter Hey guys, I have a "Milwaukee Ph 600" ph pen that has been collecting mildew on it. I was wondering if there's an easy way to get it clean and keep it clean? I bought the ph meter cleaning solution as well as the storage solution. The cleaning solution doesn't seem to clean the mildew off very well. I usually keep the meter with the cap on it and I put a small amount of the storage solution in the cap so the probe doesn't dry out. But I'm worried the mildew will contaminate the plants' water. Rating: 5