A good possibility is that by adding P.Glycol, youve simply created a perfect growth medium for a biologic growth of microorganisms. Microorganisms depending on species can essentially use any and all carbon based material as its nutrient source. What most probably you are seeing is a massive 'colony' of micro-cells that were stimulated to grow over last year's 'warm' winter.
For safety and to remove any 'pathological' species, you will have to thoroughly clean out your water system, by cleaning I mean mechanical scrubbing of the tank internals and the hose internals and not simply 'soaking' with a mild clorox, etc. solution.
Also for safety I would advise for you to wear respiratory and 'skin' protection when performing such 'cleaning'. When the system is 'mechanically' clean, then and only then you should 'sanitize' the system using 10-15 parts per million of a clorox solution and soak for at least 2-3 hours. The approximate 'recipe' of water:clorox will be 50 - 75+oz. of 'consumer grade' clorox per 100 gallons of water .... the water system 'internals' MUST be mechanically clean (scrubbing with strong detergents, etc.) before any 'sanitization' is possible as if you have a thick 'biological slime' sanitization fluids will only kill the 'surface' layers leaving viable cells down deep and which will only continue to grow and multiply. Plus, dead cells that arent 'removed' will only provide a nutrient source for other organisms.
The 'easiest' way to do this if you live in an area that has a 'dairy' industry, is to contract and pay a 'dairyman' to come and do the 'clean out', as such folks will have the knowledge and expertise of 'aggressive' chemical means to do an efficacious and almost 'sterile' clean-out.
This recommendation may seem extreme but its the 'safest' way to do this to get the tank and its piping back to a 'hygienic' state.
Since the possibility of massive 'infection' of your tank, etc. is high ... DO NOT follow the typical 'internet' recipe for a 'simple' soak with clorox ... as that 'offering' is ONLY for NEW or mechanically cleaned systems ---- NOT for systems that have developed 'bio-slimes' (massive colonies of microorganisms).
;-)