These are early Thiel accuton units i suppose ( are these Lumen white speakers , you had white flame / white light at the time
??)
They are made of aluminium oxide iirc , whether a solvent / water can be used to clean them without problems i m not sure .
If my recollection is right they are non removable grills
You could unscrew one unit and try to pull it out gently to check if it has bolts at the back to remove the grill , if its stuck i wouldnt bother as may be the wires are to short , due to soldering after mounting , it depends on the building method of the speaker
Cleaning without removing the grill is rather difficult , you could spray it on but how to wipe it off ???
If there is not a substantial weight increase due to the coloration ( fines ) then i doubt if it matters much soundwise .
Visual is another thing , i doubt also this moisture? coloration could be removed entirely .
By the way, is this a moisture that dropped on the membrane or is this a coloration of the aluminium oxide over time due to bad storage / age ( check the internet on aluminium oxide aging problems /coloration )
How long they last ?? . .... Until they break
i think the first thing to go would be the rubber? surround , but since these are high quality units , they last long.
You could inspect the rubber? surrounds for cracks , and the membranes for cracks as well .
If you play music through the speaker , you can put your ear against each unit to check if they are still operational.
I usually check if the unit is still operational by applying a ever so slight finger touch against the membrane ( not with tweeters ! ) while playing music at a low / medium level , but that aint gonna work with your grilles