Nouns / phrases:
Dwindling, deficiency, depletion. Loss, reduction, atrophy. Shortage, a lack of something, scarcity, plight - e.g. poverty, hunger, emotional neglect. Exhaustion.
Decay, spoilage, ruin. Destruction, defect, flaw. Contamination, dirt, filth.
Disease, sickness, infestation, infection, pest, plague. Something which is unhealthy or unwholesome. Harm. "Something isn't quite right!". Corruption.
Secrecy. Something is going on behind someone's back. Sneakiness; insidiousness. Rumours, slander. Dishonesty, lies, and theft.
To wane, to dwindle. To taken away, to reduce, to lower, to lose. To deplete, to drain, to waste, to exhaust. To go bad, deteriorate, decay. To gnaw at, to nibble. To destruct, ruin, spoil something. To infest, infect, attract a disease. To contaminate, foul, pollute, soil. To do something behind someone's back. To sneak, pry about, eavesdrop, spy on. To spread rumours, slander, lie, blackmail, steal.
Scarce, insufficient, poor. Lost, less of, dwindling, on the wane.
Draining, depletive. Stale, bad, deteriorating. Degenerated, foul, unswallowable. Secretive, secret. Furtive, covert, sneaky. Crooked, unfrank, disingenuous. Unwholesome, unhealthy, sick, infested,infected, contaminous, virulent. Corrupt,
corrupting. Dirty, foiled. Behind someone's back.
As a person: Energy vampire; thief. Someone who is: sneaky, corrupt. Someone who makes themselves scarce. Or: someone who has very few resources, is exhausted, poor, or sick.
Check if you have enough reserves left! Make sure it's still there! Identify your weak points/soft spots so you can protect yourself! Keep an eye out for dishonesty/corruption!
Time factor *) : Times where something is waning: waning moon, dusk, autumn. Deceleration. Or: over time something is less and less likely to happen. Too little time.