The present tense and the past tense mean different things; choosing between them here is not a matter of grammar.
As far as I know, it isn't actually clear that singular verb agreement is "grammatical" in the strict sense in this kind of context, but there is a general consensus in favor of using singular agreement, so I'd go with your intuition.
According to analyses like that of Huddleston and Pullum's CGEL, the subject of sentences like this is the word there, which is analyzed as a pronoun (there are a few arguments for this; one is the "tag question" construction "isn't there?"/"aren't there?").
In standard English, the pronoun there can take singular or plural verb agreement depending on the grammatical number of the other noun phrase in the clause.
The grammatical number of "no pus or polyps" and proximity agreement
But here we run into an issue: what is the grammatical number of "no pus or polyps"? The first part is a mass noun, and so grammatically singular: we would say "There is (no) pus". The second part is grammatically plural: we would say "There are (no) polyps". The two noun phrases are joined by "or". If they were both singular, you'd use singular agreement, and if they were both plural, you would use plural agreement, but there isn't really a clear rule in English grammar about what to do when a singular noun phrase is joined to a plural noun phrase using "or". The "compound subject" rule that you're thinking of only applies to noun phrases joined by "and".
The conventional "rule" given by guidebooks and the like for noun phrases joined by "or" is to use "proximity agreement" (which is normally considered ungrammatical) and make the verb agree with whichever noun phrase is closer. (Of course, another way out is to rephrase the sentence.)
So "There was no pus or polyps" would be considered preferable to "There were no pus or polyps", but "There were no polyps or pus" would be considered preferable to "There was no polyps or pus". (We can see how weird the "proximity agreement" rule is if we try to add a tag question: "There was no pus or polyps, were there?" and "There were no polyps or pus, was there?" sound very odd, in my opinion.)
"There is" vs. "There's"
The situation in the present tense is basically the same, except there's a third option: "There's". The contraction "There's" actually sounds more acceptable than "There is" before a plural noun phrase, so "There's no pus or polyps" sounds pretty unremarkable. However, the use of contractions may be considered informal, especially in writing, so depending on the context this might not be a good choice.
More on there as a pronoun
In fact, Nicholas Sobin has suggested that the fact that there can take plural agreement is not part of the core grammar of English, but the result of a non-core "grammatical virus" rule (see my answer to the related question Use of “Here's” before a plural noun), so if you're unsure, it's probably better to use singular agreement rather than plural agreement after there.
- Sobin, Nicholas. "Agreement, Default Rules, and Grammatical Viruses." Linguistic Inquiry Vol. 28, No. 2 (Spring, 1997), 318-343.